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Network Working Group R. Coltun
Request for Comments: 5340 Acoustra Productions
Obsoletes: 2740 D. Ferguson
Category: Standards Track Juniper Networks
J. Moy
Sycamore Networks, Inc
A. Lindem, Ed.
Redback Networks
July 2008
OSPF for IPv6
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document describes the modifications to OSPF to support version
6 of the Internet Protocol (IPv6). The fundamental mechanisms of
OSPF (flooding, Designated Router (DR) election, area support, Short
Path First (SPF) calculations, etc.) remain unchanged. However, some
changes have been necessary, either due to changes in protocol
semantics between IPv4 and IPv6, or simply to handle the increased
address size of IPv6. These modifications will necessitate
incrementing the protocol version from version 2 to version 3. OSPF
for IPv6 is also referred to as OSPF version 3 (OSPFv3).
Changes between OSPF for IPv4, OSPF Version 2, and OSPF for IPv6 as
described herein include the following. Addressing semantics have
been removed from OSPF packets and the basic Link State
Advertisements (LSAs). New LSAs have been created to carry IPv6
addresses and prefixes. OSPF now runs on a per-link basis rather
than on a per-IP-subnet basis. Flooding scope for LSAs has been
generalized. Authentication has been removed from the OSPF protocol
and instead relies on IPv6's Authentication Header and Encapsulating
Security Payload (ESP).
Even with larger IPv6 addresses, most packets in OSPF for IPv6 are
almost as compact as those in OSPF for IPv4. Most fields and packet-
size limitations present in OSPF for IPv4 have been relaxed. In
addition, option handling has been made more flexible.
Coltun, et al. Standards Track [Page 1]
RFC 5340 OSPF for IPv6 July 2008
All of OSPF for IPv4's optional capabilities, including demand
circuit support and Not-So-Stubby Areas (NSSAs), are also supported
in OSPF for IPv6.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Differences from OSPF for IPv4 . . . . . . . . . . . . . . . . 5
2.1. Protocol Processing Per-Link, Not Per-Subnet . . . . . . . 5
2.2. Removal of Addressing Semantics . . . . . . . . . . . . . 5
2.3. Addition of Flooding Scope . . . . . . . . . . . . . . . . 6
2.4. Explicit Support for Multiple Instances per Link . . . . . 6
2.5. Use of Link-Local Addresses . . . . . . . . . . . . . . . 7
2.6. Authentication Changes . . . . . . . . . . . . . . . . . . 7
2.7. Packet Format Changes . . . . . . . . . . . . . . . . . . 8
2.8. LSA Format Changes . . . . . . . . . . . . . . . . . . . . 9
2.9. Handling Unknown LSA Types . . . . . . . . . . . . . . . . 10
2.10. Stub/NSSA Area Support . . . . . . . . . . . . . . . . . . 11
2.11. Identifying Neighbors by Router ID . . . . . . . . . . . . 11
3. Differences with RFC 2740 . . . . . . . . . . . . . . . . . . 11
3.1. Support for Multiple Interfaces on the Same Link . . . . . 11
3.2. Deprecation of MOSPF for IPv6 . . . . . . . . . . . . . . 12
3.3. NSSA Specification . . . . . . . . . . . . . . . . . . . . 12
3.4. Stub Area Unknown LSA Flooding Restriction Deprecated . . 12
3.5. Link LSA Suppression . . . . . . . . . . . . . . . . . . . 12
3.6. LSA Options and Prefix Options Updates . . . . . . . . . . 13
3.7. IPv6 Site-Local Addresses . . . . . . . . . . . . . . . . 13
4. Implementation Details . . . . . . . . . . . . . . . . . . . . 13
4.1. Protocol Data Structures . . . . . . . . . . . . . . . . . 14
4.1.1. The Area Data Structure . . . . . . . . . . . . . . . 15
4.1.2. The Interface Data Structure . . . . . . . . . . . . . 15
4.1.3. The Neighbor Data Structure . . . . . . . . . . . . . 16
4.2. Protocol Packet Processing . . . . . . . . . . . . . . . . 17
4.2.1. Sending Protocol Packets . . . . . . . . . . . . . . . 17
4.2.1.1. Sending Hello Packets . . . . . . . . . . . . . . 18
4.2.1.2. Sending Database Description Packets . . . . . . . 19
4.2.2. Receiving Protocol Packets . . . . . . . . . . . . . . 19
4.2.2.1. Receiving Hello Packets . . . . . . . . . . . . . 21
4.3. The Routing table Structure . . . . . . . . . . . . . . . 22
4.3.1. Routing Table Lookup . . . . . . . . . . . . . . . . . 23
4.4. Link State Advertisements . . . . . . . . . . . . . . . . 23
4.4.1. The LSA Header . . . . . . . . . . . . . . . . . . . . 23
4.4.2. The Link-State Database . . . . . . . . . . . . . . . 24
4.4.3. Originating LSAs . . . . . . . . . . . . . . . . . . . 25
4.4.3.1. LSA Options . . . . . . . . . . . . . . . . . . . 27
4.4.3.2. Router-LSAs . . . . . . . . . . . . . . . . . . . 27
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RFC 5340 OSPF for IPv6 July 2008
4.4.3.3. Network-LSAs . . . . . . . . . . . . . . . . . . . 29
4.4.3.4. Inter-Area-Prefix-LSAs . . . . . . . . . . . . . . 30
4.4.3.5. Inter-Area-Router-LSAs . . . . . . . . . . . . . . 31
4.4.3.6. AS-External-LSAs . . . . . . . . . . . . . . . . . 32
4.4.3.7. NSSA-LSAs . . . . . . . . . . . . . . . . . . . . 33
4.4.3.8. Link-LSAs . . . . . . . . . . . . . . . . . . . . 34
4.4.3.9. Intra-Area-Prefix-LSAs . . . . . . . . . . . . . . 36
4.4.4. Future LSA Validation . . . . . . . . . . . . . . . . 40
4.5. Flooding . . . . . . . . . . . . . . . . . . . . . . . . . 40
4.5.1. Receiving Link State Update Packets . . . . . . . . . 40
4.5.2. Sending Link State Update Packets . . . . . . . . . . 41
4.5.3. Installing LSAs in the Database . . . . . . . . . . . 43
4.6. Definition of Self-Originated LSAs . . . . . . . . . . . . 43
4.7. Virtual Links . . . . . . . . . . . . . . . . . . . . . . 44
4.8. Routing Table Calculation . . . . . . . . . . . . . . . . 44
4.8.1. Calculating the Shortest-Path Tree for an Area . . . . 45
4.8.2. The Next-Hop Calculation . . . . . . . . . . . . . . . 44
4.8.3. Calculating the Inter-Area Routes . . . . . . . . . . 47
4.8.4. Examining Transit Areas' Summary-LSAs . . . . . . . . 48
4.8.5. Calculating AS External and NSSA Routes . . . . . . . 48
4.9. Multiple Interfaces to a Single Link . . . . . . . . . . . 48
4.9.1. Standby Interface State . . . . . . . . . . . . . . . 50
5. Security Considerations . . . . . . . . . . . . . . . . . . . 52
6. Manageability Considerations . . . . . . . . . . . . . . . . . 52
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 52
7.1. MOSPF for OSPFv3 Deprecation IANA Considerations . . . . . 53
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 53
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 55
9.1. Normative References . . . . . . . . . . . . . . . . . . . 55
9.2. Informative References . . . . . . . . . . . . . . . . . . 56
Appendix A. OSPF Data Formats . . . . . . . . . . . . . . . . . . 57
A.1. Encapsulation of OSPF Packets . . . . . . . . . . . . . . 57
A.2. The Options Field . . . . . . . . . . . . . . . . . . . . 58
A.3. OSPF Packet Formats . . . . . . . . . . . . . . . . . . . 60
A.3.1. The OSPF Packet Header . . . . . . . . . . . . . . . . 60
A.3.2. The Hello Packet . . . . . . . . . . . . . . . . . . . 62
A.3.3. The Database Description Packet . . . . . . . . . . . 63
A.3.4. The Link State Request Packet . . . . . . . . . . . . 65
A.3.5. The Link State Update Packet . . . . . . . . . . . . . 66
A.3.6. The Link State Acknowledgment Packet . . . . . . . . . 67
A.4. LSA Formats . . . . . . . . . . . . . . . . . . . . . . . 68
A.4.1. IPv6 Prefix Representation . . . . . . . . . . . . . . 69
A.4.1.1. Prefix Options . . . . . . . . . . . . . . . . . . 69
A.4.2. The LSA Header . . . . . . . . . . . . . . . . . . . . 70
A.4.2.1. LSA Type . . . . . . . . . . . . . . . . . . . . . 72
A.4.3. Router-LSAs . . . . . . . . . . . . . . . . . . . . . 73
A.4.4. Network-LSAs . . . . . . . . . . . . . . . . . . . . . 76
A.4.5. Inter-Area-Prefix-LSAs . . . . . . . . . . . . . . . . 77
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RFC 5340 OSPF for IPv6 July 2008
A.4.6. Inter-Area-Router-LSAs . . . . . . . . . . . . . . . . 78
A.4.7. AS-External-LSAs . . . . . . . . . . . . . . . . . . . 79
A.4.8. NSSA-LSAs . . . . . . . . . . . . . . . . . . . . . . 82
A.4.9. Link-LSAs . . . . . . . . . . . . . . . . . . . . . . 82
A.4.10. Intra-Area-Prefix-LSAs . . . . . . . . . . . . . . . . 84
Appendix B. Architectural Constants . . . . . . . . . . . . . . . 86
Appendix C. Configurable Constants . . . . . . . . . . . . . . . 86
C.1. Global Parameters . . . . . . . . . . . . . . . . . . . . 86
C.2. Area Parameters . . . . . . . . . . . . . . . . . . . . . 87
C.3. Router Interface Parameters . . . . . . . . . . . . . . . 88
C.4. Virtual Link Parameters . . . . . . . . . . . . . . . . . 90
C.5. NBMA Network Parameters . . . . . . . . . . . . . . . . . 91
C.6. Point-to-Multipoint Network Parameters . . . . . . . . . . 92
C.7. Host Route Parameters . . . . . . . . . . . . . . . . . . 92
1. Introduction
This document describes the modifications to OSPF to support version
6 of the Internet Protocol (IPv6). The fundamental mechanisms of
OSPF (flooding, Designated Router (DR) election, area support,
(Shortest Path First) SPF calculations, etc.) remain unchanged.
However, some changes have been necessary, either due to changes in
protocol semantics between IPv4 and IPv6, or simply to handle the
increased address size of IPv6. These modifications will necessitate
incrementing the protocol version from version 2 to version 3. OSPF
for IPv6 is also referred to as OSPF version 3 (OSPFv3).
This document is organized as follows. Section 2 describes the
differences between OSPF for IPv4 (OSPF version 2) and OSPF for IPv6
(OSPF version 3) in detail. Section 3 describes the difference
between RFC 2740 and this document. Section 4 provides
implementation details for the changes. Appendix A gives the OSPF
for IPv6 packet and Link State Advertisement (LSA) formats. Appendix
B lists the OSPF architectural constants. Appendix C describes
configuration parameters.
1.1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC-KEYWORDS].
1.2. Terminology
This document attempts to use terms from both the OSPF for IPv4
specification ([OSPFV2]) and the IPv6 protocol specifications
([IPV6]). This has produced a mixed result. Most of the terms used
both by OSPF and IPv6 have roughly the same meaning (e.g.,
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RFC 5340 OSPF for IPv6 July 2008
interfaces). However, there are a few conflicts. IPv6 uses "link"
similarly to IPv4 OSPF's "subnet" or "network". In this case, we
have chosen to use IPv6's "link" terminology. "Link" replaces OSPF's
"subnet" and "network" in most places in this document, although
OSPF's network-LSA remains unchanged (and possibly unfortunately, a
new link-LSA has also been created).
The names of some of the OSPF LSAs have also changed. See
Section 2.8 for details.
In the context of this document, an OSPF instance is a separate
protocol instance complete with its own protocol data structures
(e.g., areas, interfaces, neighbors), link-state database, protocol
state machines, and protocol processing (e.g., SPF calculation).
2. Differences from OSPF for IPv4
Most of the algorithms from OSPF for IPv4 [OSPFV2] have been
preserved in OSPF for IPv6. However, some changes have been
necessary, either due to changes in protocol semantics between IPv4
and IPv6, or simply to handle the increased address size of IPv6.
The following subsections describe the differences between this
document and [OSPFV2].
2.1. Protocol Processing Per-Link, Not Per-Subnet
IPv6 uses the term "link" to indicate "a communication facility or
medium over which nodes can communicate at the link layer" ([IPV6]).
"Interfaces" connect to links. Multiple IPv6 subnets can be assigned
to a single link, and two nodes can talk directly over a single link,
even if they do not share a common IPv6 subnet (IPv6 prefix).
For this reason, OSPF for IPv6 runs per-link instead of the IPv4
behavior of per-IP-subnet. The terms "network" and "subnet" used in
the IPv4 OSPF specification ([OSPFV2]) should generally be replaced
by link. Likewise, an OSPF interface now connects to a link instead
of an IP subnet.
This change affects the receiving of OSPF protocol packets, the
contents of Hello packets, and the contents of network-LSAs.
2.2. Removal of Addressing Semantics
In OSPF for IPv6, addressing semantics have been removed from the
OSPF protocol packets and the main LSA types, leaving a network-
protocol-independent core. In particular:
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RFC 5340 OSPF for IPv6 July 2008
o IPv6 addresses are not present in OSPF packets, except in LSA
payloads carried by the Link State Update packets. See
Section 2.7 for details.
o Router-LSAs and network-LSAs no longer contain network addresses,
but simply express topology information. See Section 2.8 for
details.
o OSPF Router IDs, Area IDs, and LSA Link State IDs remain at the
IPv4 size of 32 bits. They can no longer be assigned as (IPv6)
addresses.
o Neighboring routers are now always identified by Router ID.
Previously, they had been identified by an IPv4 address on
broadcast, NBMA (Non-Broadcast Multi-Access), and point-to-
multipoint links.
2.3. Addition of Flooding Scope
Flooding scope for LSAs has been generalized and is now explicitly
coded in the LSA's LS type field. There are now three separate
flooding scopes for LSAs:
o Link-local scope. LSA is only flooded on the local link and no
further. Used for the new link-LSA. See Section 4.4.3.8 for
details.
o Area scope. LSA is only flooded throughout a single OSPF area.
Used for router-LSAs, network-LSAs, inter-area-prefix-LSAs, inter-
area-router-LSAs, and intra-area-prefix-LSAs.
o AS scope. LSA is flooded throughout the routing domain. Used for
AS-external-LSAs. A router that originates AS scoped LSAs is
considered an AS Boundary Router (ASBR) and will set its E-bit in
router-LSAs for regular areas.
2.4. Explicit Support for Multiple Instances per Link
OSPF now supports the ability to run multiple OSPF protocol instances
on a single link. For example, this may be required on a NAP segment
shared between several providers. Providers may be supporting
separate OSPF routing domains that wish to remain separate even
though they have one or more physical network segments (i.e., links)
in common. In OSPF for IPv4, this was supported in a haphazard
fashion using the authentication fields in the OSPF for IPv4 header.
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RFC 5340 OSPF for IPv6 July 2008
Another use for running multiple OSPF instances is if you want, for
one reason or another, to have a single link belong to two or more
OSPF areas.
Support for multiple protocol instances on a link is accomplished via
an "Instance ID" contained in the OSPF packet header and OSPF
interface data structures. Instance ID solely affects the reception
of OSPF packets and applies to normal OSPF interfaces and virtual
links.
2.5. Use of Link-Local Addresses
IPv6 link-local addresses are for use on a single link, for purposes
of neighbor discovery, auto-configuration, etc. IPv6 routers do not
forward IPv6 datagrams having link-local source addresses [IP6ADDR].
Link-local unicast addresses are assigned from the IPv6 address range
FE80/10.
OSPF for IPv6 assumes that each router has been assigned link-local
unicast addresses on each of the router's attached physical links
[IP6ADDR]. On all OSPF interfaces except virtual links, OSPF packets
are sent using the interface's associated link-local unicast address
as the source address. A router learns the link-local addresses of
all other routers attached to its links and uses these addresses as
next-hop information during packet forwarding.
On virtual links, a global scope IPv6 address MUST be used as the
source address for OSPF protocol packets.
Link-local addresses appear in OSPF link-LSAs (see Section 4.4.3.8).
However, link-local addresses are not allowed in other OSPF LSA
types. In particular, link-local addresses MUST NOT be advertised in
inter-area-prefix-LSAs (Section 4.4.3.4), AS-external-LSAs
(Section 4.4.3.6), NSSA-LSAs (Section 4.4.3.7), or intra-area-prefix-
LSAs (Section 4.4.3.9).
2.6. Authentication Changes
In OSPF for IPv6, authentication has been removed from the OSPF
protocol. The "AuType" and "Authentication" fields have been removed
from the OSPF packet header, and all authentication-related fields
have been removed from the OSPF area and interface data structures.
When running over IPv6, OSPF relies on the IP Authentication Header
(see [IPAUTH]) and the IP Encapsulating Security Payload (see
[IPESP]) as described in [OSPFV3-AUTH] to ensure integrity and
authentication/confidentiality of routing exchanges.
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RFC 5340 OSPF for IPv6 July 2008
Protection of OSPF packet exchanges against accidental data
corruption is provided by the standard IPv6 Upper-Layer checksum (as
described in Section 8.1 of [IPV6]), covering the entire OSPF packet
and prepended IPv6 pseudo-header (see Appendix A.3.1).
2.7. Packet Format Changes
OSPF for IPv6 runs directly over IPv6. Aside from this, all
addressing semantics have been removed from the OSPF packet headers,
making it essentially "network-protocol-independent". All addressing
information is now contained in the various LSA types only.
In detail, changes in OSPF packet format consist of the following:
o The OSPF version number has been incremented from 2 to 3.
o The Options field in Hello packets and Database Description
packets has been expanded to 24 bits.
o The Authentication and AuType fields have been removed from the
OSPF packet header (see Section 2.6).
o The Hello packet now contains no address information at all.
Rather, it now includes an Interface ID that the originating
router has assigned to uniquely identify (among its own
interfaces) its interface to the link. This Interface ID will be
used as the network-LSA's Link State ID if the router becomes the
Designated Router on the link.
o Two Options bits, the "R-bit" and the "V6-bit", have been added to
the Options field for processing router-LSAs during the SPF
calculation (see Appendix A.2). If the "R-bit" is clear, an OSPF
speaker can participate in OSPF topology distribution without
being used to forward transit traffic; this can be used in multi-
homed hosts that want to participate in the routing protocol. The
V6-bit specializes the R-bit; if the V6-bit is clear, an OSPF
speaker can participate in OSPF topology distribution without
being used to forward IPv6 datagrams. If the R-bit is set and the
V6-bit is clear, IPv6 datagrams are not forwarded but datagrams
belonging to another protocol family may be forwarded.
o The OSPF packet header now includes an "Instance ID" that allows
multiple OSPF protocol instances to be run on a single link (see
Section 2.4).
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2.8. LSA Format Changes
All addressing semantics have been removed from the LSA header,
router-LSAs, and network-LSAs. These two LSAs now describe the
routing domain's topology in a network-protocol-independent manner.
New LSAs have been added to distribute IPv6 address information and
data required for next-hop resolution. The names of some of IPv4's
LSAs have been changed to be more consistent with each other.
In detail, changes in LSA format consist of the following:
o The Options field has been removed from the LSA header, expanded
to 24 bits, and moved into the body of router-LSAs, network-LSAs,
inter-area-router-LSAs, and link-LSAs. See Appendix A.2 for
details.
o The LSA Type field has been expanded (into the former Options
space) to 16 bits, with the upper three bits encoding flooding
scope and the handling of unknown LSA types (see Section 2.9).
o Addresses in LSAs are now expressed as [prefix, prefix length]
instead of [address, mask] (see Appendix A.4.1). The default
route is expressed as a prefix with length 0.
o Router-LSAs and network-LSAs now have no address information and
are network protocol independent.
o Router interface information MAY be spread across multiple router-
LSAs. Receivers MUST concatenate all the router-LSAs originated
by a given router when running the SPF calculation.
o A new LSA called the link-LSA has been introduced. Link-LSAs have
link-local flooding scope; they are never flooded beyond the link
with which they are associated. Link-LSAs have three purposes: 1)
they provide the router's link-local address to all other routers
attached to the link, 2) they inform other routers attached to the
link of a list of IPv6 prefixes to associate with the link, and 3)
they allow the router to advertise a collection of Options bits to
associate with the network-LSA that will be originated for the
link. See Section 4.4.3.8 for details.
o In IPv4, the router-LSA carries a router's IPv4 interface
addresses, the IPv4 equivalent of link-local addresses. These are
only used when calculating next hops during the OSPF routing
calculation (see Section 16.1.1 of [OSPFV2]), so they do not need
to be flooded past the local link. Hence, using link-LSAs to
distribute these addresses is more efficient. Note that link-
local addresses cannot be learned through the reception of Hellos
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RFC 5340 OSPF for IPv6 July 2008
in all cases. On NBMA links, next-hop routers do not necessarily
exchange Hellos. Rather, these routers learn of each other's
existence by way of the Designated Router (DR).
o The Options field in the network LSA is set to the logical OR of
the Options that each router on the link advertises in its link-
LSA.
o Type-3 summary-LSAs have been renamed "inter-area-prefix-LSAs".
Type-4 summary LSAs have been renamed "inter-area-router-LSAs".
o The Link State ID in inter-area-prefix-LSAs, inter-area-router-
LSAs, NSSA-LSAs, and AS-external-LSAs has lost its addressing
semantics and now serves solely to identify individual pieces of
the Link State Database. All addresses or Router IDs that were
formerly expressed by the Link State ID are now carried in the LSA
bodies.
o Network-LSAs and link-LSAs are the only LSAs whose Link State ID
carries additional meaning. For these LSAs, the Link State ID is
always the Interface ID of the originating router on the link
being described. For this reason, network-LSAs and link-LSAs are
now the only LSAs whose size cannot be limited: a network-LSA MUST
list all routers connected to the link and a link-LSA MUST list
all of a router's addresses on the link.
o A new LSA called the intra-area-prefix-LSA has been introduced.
This LSA carries all IPv6 prefix information that in IPv4 is
included in router-LSAs and network-LSAs. See Section 4.4.3.9 for
details.
o Inclusion of a forwarding address or external route tag in AS-
external-LSAs is now optional. In addition, AS-external-LSAs can
now reference another LSA, for inclusion of additional route
attributes that are outside the scope of the OSPF protocol. For
example, this reference could be used to attach BGP path
attributes to external routes.
2.9. Handling Unknown LSA Types
Handling of unknown LSA types has been made more flexible so that,
based on the LS type, unknown LSA types are either treated as having
link-local flooding scope, or are stored and flooded as if they were
understood. This behavior is explicitly coded in the LSA Handling
bit of the link state header's LS type field (see the U-bit in
Appendix A.4.2.1).
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The IPv4 OSPF behavior of simply discarding unknown types is
unsupported due to the desire to mix router capabilities on a single
link. Discarding unknown types causes problems when the Designated
Router supports fewer options than the other routers on the link.
2.10. Stub/NSSA Area Support
In OSPF for IPv4, stub and NSSA areas were designed to minimize link-
state database and routing table sizes for the areas' internal
routers. This allows routers with minimal resources to participate
in even very large OSPF routing domains.
In OSPF for IPv6, the concept of stub and NSSA areas is retained. In
IPv6, of the mandatory LSA types, stub areas carry only router-LSAs,
network-LSAs, inter-area-prefix-LSAs, link-LSAs, and intra-area-
prefix-LSAs. NSSA areas are restricted to these types and, of
course, NSSA-LSAs. This is the IPv6 equivalent of the LSA types
carried in IPv4 stub areas: router-LSAs, network-LSAs, type 3
summary-LSAs and for NSSA areas: stub area types and NSSA-LSAs.
2.11. Identifying Neighbors by Router ID
In OSPF for IPv6, neighboring routers on a given link are always
identified by their OSPF Router ID. This contrasts with the IPv4
behavior where neighbors on point-to-point networks and virtual links
are identified by their Router IDs while neighbors on broadcast,
NBMA, and point-to-multipoint links are identified by their IPv4
interface addresses.
This change affects the reception of OSPF packets (see Section 8.2 of
[OSPFV2]), the lookup of neighbors (Section 10 of [OSPFV2]), and the
reception of Hello packets (Section 10.5 of [OSPFV2]).
The Router ID of 0.0.0.0 is reserved and SHOULD NOT be used.
3. Differences with RFC 2740
OSPFv3 implementations based on RFC 2740 will fully interoperate with
implementations based on this specification. There are, however,
some protocol additions and changes (all of which are backward
compatible).
3.1. Support for Multiple Interfaces on the Same Link
This protocol feature was only partially specified in the RFC 2740.
The level of specification was insufficient to implement the feature.
Section 4.9 specifies the additions and clarifications necessary for
implementation. They are fully compatible with RFC 2740.
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3.2. Deprecation of MOSPF for IPv6
This protocol feature was only partially specified in RFC 2740. The
level of specification was insufficient to implement the feature.
There are no known implementations. Multicast Extensions to OSPF
(MOSPF) support and its attendant protocol fields have been
deprecated from OSPFv3. Refer to Section 4.4.3.2, Section 4.4.3.4,
Section 4.4.3.6, Section 4.4.3.7, Appendix A.2, Appendix A.4.2.1,
Appendix A.4.3, Appendix A.4.1.1, and Section 7.1.
3.3. NSSA Specification
This protocol feature was only partially specified in RFC 2740. The
level of specification was insufficient to implement the function.
This document includes an NSSA specification unique to OSPFv3. This
specification coupled with [NSSA] provide sufficient specification
for implementation. Refer to Section 4.8.5, Appendix A.4.3,
Appendix A.4.8, and [NSSA].
3.4. Stub Area Unknown LSA Flooding Restriction Deprecated
In RFC 2740 [OSPFV3], flooding of unknown LSA was restricted within
stub and NSSA areas. The text describing this restriction is
included below.
However, unlike in IPv4, IPv6 allows LSAs with unrecognized
LS types to be labeled "Store and flood the LSA, as if type
understood" (see the U-bit in Appendix A.4.2.1). Uncontrolled
introduction of such LSAs could cause a stub area's link-state
database to grow larger than its component routers' capacities.
To guard against this, the following rule regarding stub areas
has been established: an LSA whose LS type is unrecognized can
only be flooded into/throughout a stub area if both a) the LSA
has area or link-local flooding scope and b) the LSA has U-bit
set to 0. See Section 3.5 for details.
This restriction has been deprecated. OSPFv3 routers will flood link
and area scope LSAs whose LS type is unrecognized and whose U-bit is
set to 1 throughout stub and NSSA areas. There are no backward-
compatibility issues other than OSPFv3 routers still supporting the
restriction may not propagate newly defined LSA types.
3.5. Link LSA Suppression
The LinkLSASuppression interface configuration parameter has been
added. If LinkLSASuppression is configured for an interface and the
interface type is not broadcast or NBMA, origination of the link-LSA
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may be suppressed. The LinkLSASuppression interface configuration
parameter is described in Appendix C.3. Section 4.8.2 and
Section 4.4.3.8 were updated to reflect the parameter's usage.
3.6. LSA Options and Prefix Options Updates
The LSA Options and Prefix Options fields have been updated to
reflect recent protocol additions. Specifically, bits related to
MOSPF have been deprecated, Options field bits common with OSPFv2
have been reserved, and the DN-bit has been added to the prefix-
options. Refer to Appendix A.2 and Appendix A.4.1.1.
3.7. IPv6 Site-Local Addresses
All references to IPv6 site-local addresses have been removed.
4. Implementation Details
When going from IPv4 to IPv6, the basic OSPF mechanisms remain
unchanged from those documented in [OSPFV2]. These mechanisms are
briefly outlined in Section 4 of [OSPFV2]. Both IPv6 and IPv4 have a
link-state database composed of LSAs and synchronized between
adjacent routers. Initial synchronization is performed through the
Database Exchange process, which includes the exchange of Database
Description, Link State Request, and Link State Update packets.
Thereafter, database synchronization is maintained via flooding,
utilizing Link State Update and Link State Acknowledgment packets.
Both IPv6 and IPv4 use OSPF Hello packets to discover and maintain
neighbor relationships, as well as to elect Designated Routers and
Backup Designated Routers on broadcast and NBMA links. The decision
as to which neighbor relationships become adjacencies, and the basic
ideas behind inter-area routing, importing external information in
AS-external-LSAs, and the various routing calculations are also the
same.
In particular, the following IPv4 OSPF functionality described in
[OSPFV2] remains completely unchanged for IPv6:
o Both IPv4 and IPv6 use OSPF packet types described in Section 4.3
of [OSPFV2], namely: Hello, Database Description, Link State
Request, Link State Update, and Link State Acknowledgment packets.
While in some cases (e.g., Hello packets) their format has changed
somewhat, the functions of the various packet types remain the
same.
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o The system requirements for an OSPF implementation remain
unchanged, although OSPF for IPv6 requires an IPv6 protocol stack
(from the network layer on down) since it runs directly over the
IPv6 network layer.
o The discovery and maintenance of neighbor relationships, and the
selection and establishment of adjacencies, remain the same. This
includes election of the Designated Router and Backup Designated
Router on broadcast and NBMA links. These mechanisms are
described in Sections 7, 7.1, 7.2, 7.3, 7.4, and 7.5 of [OSPFV2].
o The link types (or equivalently, interface types) supported by
OSPF remain unchanged, namely: point-to-point, broadcast, NBMA,
point-to-multipoint, and virtual links.
o The interface state machine, including the list of OSPF interface
states and events, and the Designated Router and Backup Designated
Router election algorithm remain unchanged. These are described
in Sections 9.1, 9.2, 9.3, and 9.4 of [OSPFV2].
o The neighbor state machine, including the list of OSPF neighbor
states and events, remains unchanged. The neighbor state machine
is described in Sections 10.1, 10.2, 10.3, and 10.4 of [OSPFV2].
o Aging of the link-state database, as well as flushing LSAs from
the routing domain through the premature aging process, remains
unchanged from the description in Sections 14 and 14.1 of
[OSPFV2].
However, some OSPF protocol mechanisms have changed as previously
described in Section 2 herein. These changes are explained in detail
in the following subsections, making references to the appropriate
sections of [OSPFV2].
The following subsections provide a recipe for turning an IPv4 OSPF
implementation into an IPv6 OSPF implementation.
4.1. Protocol Data Structures
The major OSPF data structures are the same for both IPv4 and IPv6:
areas, interfaces, neighbors, the link-state database, and the
routing table. The top-level data structures for IPv6 remain those
listed in Section 5 of [OSPFV2], with the following modifications:
o All LSAs with known LS type and AS flooding scope appear in the
top-level data structure, instead of belonging to a specific area
or link. AS-external-LSAs are the only LSAs defined by this
specification that have AS flooding scope. LSAs with unknown LS
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type, U-bit set to 1 (flood even when unrecognized), and AS
flooding scope also appear in the top-level data structure.
4.1.1. The Area Data Structure
The IPv6 area data structure contains all elements defined for IPv4
areas in Section 6 of [OSPFV2]. In addition, all LSAs of known type
that have area flooding scope are contained in the IPv6 area data
structure. This always includes the following LSA types: router-
LSAs, network-LSAs, inter-area-prefix-LSAs, inter-area-router-LSAs,
and intra-area-prefix-LSAs. LSAs with unknown LS type, U-bit set to
1 (flood even when unrecognized), and area scope also appear in the
area data structure. NSSA-LSAs are also included in an NSSA area's
data structure.
4.1.2. The Interface Data Structure
In OSPF for IPv6, an interface connects a router to a link. The IPv6
interface structure modifies the IPv4 interface structure (as defined
in Section 9 of [OSPFV2]) as follows:
Interface ID
Every interface is assigned an Interface ID, which uniquely
identifies the interface with the router. For example, some
implementations MAY be able to use the MIB-II IfIndex ([INTFMIB])
as the Interface ID. The Interface ID appears in Hello packets
sent out the interface, the link-local-LSA originated by the
router for the attached link, and the router-LSA originated by the
router-LSA for the associated area. It will also serve as the
Link State ID for the network-LSA that the router will originate
for the link if the router is elected Designated Router.
The Interface ID for a virtual link is independent of the
Interface ID of the outgoing interface it traverses in the transit
area.
Instance ID
Every interface is assigned an Instance ID. This should default
to 0. It is only necessary to assign a value other than 0 on
those links that will contain multiple separate communities of
OSPF routers. For example, suppose that there are two communities
of routers on a given ethernet segment that you wish to keep
separate.
The first community is assigned an Instance ID of 0 and all the
routers in the first community will be assigned 0 as the Instance
ID for interfaces connected to the ethernet segment. An Instance
ID of 1 is assigned to the other routers' interfaces connected to
the ethernet segment. The OSPF transmit and receive processing
(see Section 4.2) will then keep the two communities separate.
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List of LSAs with link-local scope
All LSAs with link-local scope and that were originated/flooded on
the link belong to the interface structure that connects to the
link. This includes the collection of the link's link-LSAs.
IP interface address
For IPv6, the IPv6 address appearing in the source of OSPF packets
sent on the interface is almost always a link-local address. The
one exception is for virtual links that MUST use one of the
router's own global IPv6 addresses as IP interface address.
List of link prefixes
A list of IPv6 prefixes can be configured for the attached link.
These will be advertised by the router in link-LSAs, so that they
can be advertised by the link's Designated Router in intra-area-
prefix-LSAs.
In OSPF for IPv6, each router interface has a single metric
representing the cost of sending packets on the interface. In
addition, OSPF for IPv6 relies on the IP Authentication Header (see
[IPAUTH]) and the IP Encapsulating Security Payload (see [IPESP]) as
described in [OSPFV3-AUTH] to ensure integrity and authentication/
confidentiality of routing exchanges. For this reason, AuType and
Authentication key are not associated with IPv6 OSPF interfaces.
Interface states, events, and the interface state machine remain
unchanged from IPv4 as documented in Sections 9.1, 9.2, and 9.3 of
[OSPFV2] respectively. The Designated Router and Backup Designated
Router election algorithm also remains unchanged from the IPv4
election in Section 9.4 of [OSPFV2].
4.1.3. The Neighbor Data Structure
The neighbor structure performs the same function in both IPv6 and
IPv4. Namely, it collects all information required to form an
adjacency between two routers when such an adjacency becomes
necessary. Each neighbor structure is bound to a single OSPF
interface. The differences between the IPv6 neighbor structure and
the neighbor structure defined for IPv4 in Section 10 of [OSPFV2]
are:
Neighbor's Interface ID
The Interface ID that the neighbor advertises in its Hello packets
must be recorded in the neighbor structure. The router will
include the neighbor's Interface ID in the router's router-LSA
when either a) advertising a point-to-point or point-to-multipoint
link to the neighbor or b) advertising a link to a network where
the neighbor has become the Designated Router.
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Neighbor IP address
The neighbor's IPv6 address contained as the source address in
OSPF for IPv6 packets. This will be an IPv6 link-local address
for all link types except virtual links.
Neighbor's Designated Router
The neighbor's choice of Designated Router is now encoded as a
Router ID instead of as an IP address.
Neighbor's Backup Designated Router
The neighbor's choice of Backup Designated Router is now encoded
as a Router ID instead of as an IP address.
Neighbor states, events, and the neighbor state machine remain
unchanged from IPv4 as documented in Sections 10.1, 10.2, and 10.3 of
[OSPFV2] respectively. The decision as to which adjacencies to form
also remains unchanged from the IPv4 logic documented in Section 10.4
of [OSPFV2].
4.2. Protocol Packet Processing
OSPF for IPv6 runs directly over IPv6's network layer. As such, it
is encapsulated in one or more IPv6 headers with the Next Header
field of the immediately encapsulating IPv6 header set to the value
89.
As for OSPF for IPv4, OSPF for IPv6 OSPF routing protocol packets are
sent along adjacencies only (with the exception of Hello packets,
which are used to discover the adjacencies). OSPF packet types and
functions are the same in both IPv4 and IPv6, encoded by the Type
field of the standard OSPF packet header.
4.2.1. Sending Protocol Packets
When an IPv6 router sends an OSPF routing protocol packet, it fills
in the fields of the standard OSPF for IPv6 packet header (see
Appendix A.3.1) as follows:
Version #
Set to 3, the version number of the protocol as documented in this
specification.
Type
The type of OSPF packet, such as Link State Update or Hello
packet.
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Packet length
The length of the entire OSPF packet in bytes, including the
standard OSPF packet header.
Router ID
The identity of the router itself (who is originating the packet).
Area ID
The OSPF area for the interface on which the packet is being sent.
Instance ID
The OSPF Instance ID associated with the interface out of which
the packet is being sent.
Checksum
The standard IPv6 Upper-Layer checksum (as described in Section
8.1 of [IPV6]) covering the entire OSPF packet and prepended IPv6
pseudo-header (see Appendix A.3.1).
Selection of OSPF routing protocol packets' IPv6 source and
destination addresses is performed identically to the IPv4 logic in
Section 8.1 of [OSPFV2]. The IPv6 destination address is chosen from
among the addresses AllSPFRouters, AllDRouters, and the Neighbor IP
address associated with the other end of the adjacency (which in
IPv6, for all links except virtual links, is an IPv6 link-local
address).
The sending of Link State Request packets and Link State
Acknowledgment packets remains unchanged from the IPv4 procedures
documented in Sections 10.9 and 13.5 of [OSPFV2] respectively.
Sending Hello packets is documented in Section 4.2.1.1, and the
sending of Database Description packets in Section 4.2.1.2. The
sending of Link State Update packets is documented in Section 4.5.2.
4.2.1.1. Sending Hello Packets
IPv6 changes the way OSPF Hello packets are sent in the following
ways (compare to Section 9.5 of [OSPFV2]):
o Before the Hello packet is sent on an interface, the interface's
Interface ID MUST be copied into the Hello packet.
o The Hello packet no longer contains an IP network mask since OSPF
for IPv6 runs per-link instead of per-subnet.
o The choice of Designated Router and Backup Designated Router is
now indicated within Hellos by their Router IDs instead of by
their IP interface addresses. Advertising the Designated Router
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(or Backup Designated Router) as 0.0.0.0 indicates that the
Designated Router (or Backup Designated Router) has not yet been
chosen.
o The Options field within Hello packets has moved around, getting
larger in the process. More Options bits are now possible. Those
that MUST be set correctly in Hello packets are as follows. The
E-bit is set if and only if the interface attaches to a regular
area, i.e., not a stub or NSSA area. Similarly, the N-bit is set
if and only if the interface attaches to an NSSA area (see
[NSSA]). Finally, the DC-bit is set if and only if the router
wishes to suppress the sending of future Hellos over the interface
(see [DEMAND]). Unrecognized bits in the Hello packet's Options
field should be cleared.
Sending Hello packets on NBMA networks proceeds for IPv6 in exactly
the same way as for IPv4, as documented in Section 9.5.1 of [OSPFV2].
4.2.1.2. Sending Database Description Packets
The sending of Database Description packets differs from Section 10.8
of [OSPFV2] in the following ways:
o The Options field within Database Description packets has moved
around, getting larger in the process. More Options bits are now
possible. Those that MUST be set correctly in Database
Description packets are as follows. The DC-bit is set if and only
if the router wishes to suppress the sending of Hellos over the
interface (see [DEMAND]). Unrecognized bits in the Database
Description packet's Options field should be cleared.
4.2.2. Receiving Protocol Packets
Whenever a router receives an OSPF protocol packet, it is marked with
the interface on which it was received. For routers that have
virtual links configured, it may not be immediately obvious with
which interface to associate the packet. For example, consider the
Router RT11 depicted in Figure 6 of [OSPFV2]. If RT11 receives an
OSPF protocol packet on its interface to Network N8, it may want to
associate the packet with the interface to Area 2, or with the
virtual link to Router RT10 (which is part of the backbone). In the
following, we assume that the packet is initially associated with the
non-virtual link.
In order for the packet to be passed to OSPF for processing, the
following tests must be performed on the encapsulating IPv6 headers:
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o The packet's IP destination address MUST be one of the IPv6
unicast addresses associated with the receiving interface (this
includes link-local addresses), one of the IPv6 multicast
addresses AllSPFRouters or AllDRouters, or an IPv6 global address
(for virtual links).
o The Next Header field of the immediately encapsulating IPv6 header
MUST specify the OSPF protocol (89).
o Any encapsulating IP Authentication Headers (see [IPAUTH]) and the
IP Encapsulating Security Payloads (see [IPESP]) MUST be processed
and/or verified to ensure integrity and authentication/
confidentiality of OSPF routing exchanges. This is described in
[OSPFV3-AUTH].
After processing the encapsulating IPv6 headers, the OSPF packet
header is processed. The fields specified in the header must match
those configured for the receiving OSPFv3 interface. If they do not,
the packet SHOULD be discarded:
o The version number field MUST specify protocol version 3.
o The IPv6 Upper-Layer checksum (as described in Section 8.1 of
[IPV6]), covering the entire OSPF packet and prepended IPv6
pseudo-header, must be verified (see Appendix A.3.1).
o The Area ID and Instance ID found in the OSPF header must be
verified. If both of the following cases fail, the packet should
be discarded. The Area ID and Instance ID specified in the header
must either:
1. Match one of the Area ID(s) and Interface Instance ID(s) for
the receiving link. Unlike IPv4, the IPv6 source address is
not restricted to lie within the same IPv6 subnet as the
receiving link. IPv6 OSPF runs per-link instead of per-IP-
subnet.
2. Match the backbone area and other criteria for a configured
virtual link. The receiving router must be an ABR (Area
Border Router) and the Router ID specified in the packet (the
source router) must be the other end of a configured virtual
link. Additionally, the receiving link must have an OSPFv3
interface that attaches to the virtual link's configured
transit area and the Instance ID must match the virtual link's
Instance ID. If all of these checks succeed, the packet is
accepted and is associated with the virtual link (and the
backbone area).
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o Locally originated packets SHOULD NOT be processed by OSPF except
for support of multiple interfaces attached to the same link as
described in Section 4.9. Locally originated packets have a
source address equal to one of the router's local addresses.
o Packets whose IPv6 destination is AllDRouters should only be
accepted if the state of the receiving OSPFv3 interface is DR or
Backup (see Section 9.1 [OSPFV2]).
After header processing, the packet is further processed according to
its OSPF packet type. OSPF packet types and functions are the same
for both IPv4 and IPv6.
If the packet type is Hello, it should then be further processed by
the Hello packet processing as described in Section 4.2.2.1. All
other packet types are sent/received only on adjacencies. This means
that the packet must have been sent by one of the router's active
neighbors. The neighbor is identified by the Router ID appearing in
the received packet's OSPF header. Packets not matching any active
neighbor are discarded.
The receive processing of Database Description packets, Link State
Request packets, and Link State Acknowledgment packets is almost
identical to the IPv4 procedures documented in Sections 10.6, 10.7,
and 13.7 of [OSPFV2] respectively with the exceptions noted below.
o LSAs with unknown LS types in Database Description packets that
have an acceptable flooding scope are processed the same as LSAs
with known LS types. In OSPFv2 [OSPFV2], these would result in
the adjacency being brought down with a SequenceMismatch event.
The receiving of Hello packets is documented in Section 4.2.2.1 and
the receiving of Link State Update packets is documented in
Section 4.5.1.
4.2.2.1. Receiving Hello Packets
The receive processing of Hello packets differs from Section 10.5 of
[OSPFV2] in the following ways:
o On all link types (e.g., broadcast, NBMA, point-to-point, etc.),
neighbors are identified solely by their OSPF Router ID. For all
link types except virtual links, the Neighbor IP address is set to
the IPv6 source address in the IPv6 header of the received OSPF
Hello packet.
o There is no longer a Network Mask field in the Hello packet.
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o The neighbor's choice of Designated Router and Backup Designated
Router is now encoded as an OSPF Router ID instead of an IP
interface address.
4.3. The Routing table Structure
The routing table used by OSPF for IPv4 is defined in Section 11 of
[OSPFV2]. For IPv6, there are analogous routing table entries: there
are routing table entries for IPv6 address prefixes and also for AS
boundary routers. The latter routing table entries are only used to
hold intermediate results during the routing table build process (see
Section 4.8).
Also, to hold the intermediate results during the shortest-path
calculation for each area, there is a separate routing table for each
area holding the following entries:
o An entry for each router in the area. Routers are identified by
their OSPF Router ID. These routing table entries hold the set of
shortest paths through a given area to a given router, which in
turn allows calculation of paths to the IPv6 prefixes advertised
by that router in intra-area-prefix-LSAs. If the router is also
an area border router, these entries are also used to calculate
paths for inter-area address prefixes. If in addition the router
is the other endpoint of a virtual link, the routing table entry
describes the cost and viability of the virtual link.
o An entry for each transit link in the area. Transit links have
associated network-LSAs. Both the transit link and the network-
LSA are identified by a combination of the Designated Router's
Interface ID on the link and the Designated Router's OSPF Router
ID. These routing table entries allow later calculation of paths
to IP prefixes advertised for the transit link in intra-area-
prefix-LSAs.
The fields in the IPv4 OSPF routing table (see Section 11 of
[OSPFV2]) remain valid for IPv6: optional capabilities (routers
only), path type, cost, type 2 cost, link state origin, and for each
of the equal cost paths to the destination, the next-hop and
advertising routers.
For IPv6, the link-state origin field in the routing table entry is
the router-LSA or network-LSA that has directly or indirectly
produced the routing table entry. For example, if the routing table
entry describes a route to an IPv6 prefix, the link state origin is
the router-LSA or network-LSA that is listed in the body of the
intra-area-prefix-LSA that has produced the route (see
Appendix A.4.10).
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4.3.1. Routing Table Lookup
Routing table lookup (i.e., determining the best matching routing
table entry during IP forwarding) is the same for IPv6 as for IPv4.
4.4. Link State Advertisements
For IPv6, the OSPF LSA header has changed slightly, with the LS type
field expanding and the Options field being moved into the body of
appropriate LSAs. Also, the formats of some LSAs have changed
somewhat (namely, router-LSAs, network-LSAs, AS-external-LSAs, and
NSSA-LSAs), while the names of other LSAs have been changed (type 3
and 4 summary-LSAs are now inter-area-prefix-LSAs and inter-area-
router-LSAs respectively) and additional LSAs have been added (link-
LSAs and intra-area-prefix-LSAs). Type of Service (TOS) has been
removed from the OSPFv2 specification [OSPFV2] and is not encoded
within OSPF for IPv6's LSAs.
These changes will be described in detail in the following
subsections.
4.4.1. The LSA Header
In both IPv4 and IPv6, all OSPF LSAs begin with a standard 20-byte
LSA header. However, the contents of this 20-byte header have
changed in IPv6. The LS age, Advertising Router, LS Sequence Number,
LS checksum, and length fields within the LSA header remain
unchanged, as documented in Sections 12.1.1, 12.1.5, 12.1.6, 12.1.7,
and A.4.1 of [OSPFV2], respectively. However, the following fields
have changed for IPv6:
Options
The Options field has been removed from the standard 20-byte LSA
header and moved into the body of router-LSAs, network-LSAs,
inter-area-router-LSAs, and link-LSAs. The size of the Options
field has increased from 8 to 24 bits, and some of the bit
definitions have changed (see Appendix A.2). Additionally, a
separate PrefixOptions field, 8 bits in length, is attached to
each prefix advertised within the body of an LSA.
LS type
The size of the LS type field has increased from 8 to 16 bits,
with high-order bit encoding the handling of unknown types and the
next two bits encoding flooding scope. See Appendix A.4.2.1 for
the current coding of the LS type field.
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Link State ID
The Link State ID remains at 32 bits in length. However, except
for network-LSAs and link-LSAs, the Link State ID has shed any
addressing semantics. For example, an IPv6 router originating
multiple AS-external-LSAs could start by assigning the first a
Link State ID of 0.0.0.1, the second a Link State ID of 0.0.0.2,
and so on. Instead of the IPv4 behavior of encoding the network
number within the AS-external-LSA's Link State ID, the IPv6 Link
State ID simply serves as a way to differentiate multiple LSAs
originated by the same router.
For network-LSAs, the Link State ID is set to the Designated
Router's Interface ID on the link. When a router originates a
link-LSA for a given link, its Link State ID is set equal to the
router's Interface ID on the link.
4.4.2. The Link-State Database
In IPv6, as in IPv4, individual LSAs are identified by a combination
of their LS type, Link State ID, and Advertising Router fields.
Given two instances of an LSA, the most recent instance is determined
by examining the LSAs' LS sequence number, using LS checksum and LS
age as tiebreakers (see Section 13.1 of [OSPFV2]).
In IPv6, the link-state database is split across three separate data
structures. LSAs with AS flooding scope are contained within the
top-level OSPF data structure (see Section 4.1) as long as either
their LS type is known or their U-bit is 1 (flood even when
unrecognized); this includes the AS-external-LSAs. LSAs with area
flooding scope are contained within the appropriate area structure
(see Section 4.1.1) as long as either their LS type is known or their
U-bit is 1 (flood even when unrecognized); this includes router-LSAs,
network-LSAs, inter-area-prefix-LSAs, inter-area-router-LSAs, NSSA-
LSAs, and intra-area-prefix-LSAs. LSAs with an unknown LS type, the
U-bit set to 0, and/or link-local flooding scope are contained within
the appropriate interface structure (see Section 4.1.2); this
includes link-LSAs.
To look up or install an LSA in the database, you first examine the
LS type and the LSA's context (i.e., the area or link to which the
LSA belongs). This information allows you to find the correct
database of LSAs where you then search based on the LSA's type, Link
State ID, and Advertising Router.
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RFC 5340 OSPF for IPv6 July 2008
4.4.3. Originating LSAs
The process of reoriginating an LSA in IPv6 is the same as in IPv4:
the LSA's LS sequence number is incremented, its LS age is set to 0,
its LS checksum is calculated, and the LSA is added to the link state
database and flooded on the appropriate interfaces.
The list of events causing LSAs to be reoriginated for IPv4 is given
in Section 12.4 of [OSPFV2]. The following events and/or actions are
added for IPv6:
o The state or interface ID of one of the router's interfaces
changes. The router may need to (re)originate or flush its link-
LSA and one or more router-LSAs and/or intra-area-prefix-LSAs. If
the router is the Designated Router, the router may also need to
(re)originate and/or flush the network-LSA corresponding to the
interface.
o The identity of a link's Designated Router changes. The router
may need to (re)originate or flush the link's network-LSA and one
or more router-LSAs and/or intra-area-prefix-LSAs.
o A neighbor transitions to/from "Full" state. The router may need
to (re)originate or flush the link's network-LSA and one or more
router-LSAs and/or intra-area-prefix-LSAs.
o The Interface ID of a neighbor changes. This may cause a new
instance of a router-LSA to be originated for the associated area.
o A new prefix is added to an attached link, or a prefix is deleted
(both through configuration). This causes the router to
reoriginate its link-LSA for the link or, if it is the only router
attached to the link, causes the router to reoriginate an intra-
area-prefix-LSA.
o A new link-LSA is received, causing the link's collection of
prefixes to change. If the router is the Designated Router for
the link, it originates a new intra-area-prefix-LSA.
o A new link-LSA is received, causing the logical OR of LSA options
advertised by adjacent routers on the link to change. If the
router is the Designated Router for the link, it originates a new
network-LSA.
Detailed construction of the seven required IPv6 LSA types is
supplied by the following subsections. In order to display example
LSAs, the network map in Figure 15 of [OSPFV2] has been reworked to
show IPv6 addressing, resulting in Figure 1. The OSPF cost of each
Coltun, et al. Standards Track [Page 25]
RFC 5340 OSPF for IPv6 July 2008
interface is displayed in Figure 1. The assignment of IPv6 prefixes
to network links is shown in Table 1. A single area address range
has been configured for Area 1, so that outside of Area 1 all of its
prefixes are covered by a single route to 2001:0db8:c001::/48. The
OSPF interface IDs and the link-local addresses for the router
interfaces in Figure 1 are given in Table 2.
..........................................
. Area 1.
. + .
. | .
. | 3+---+1 .
. N1 |--|RT1|-----+ .
. | +---+ \ .
. | \ ______ .
. + \/ \ 1+---+
. * N3 *------|RT4|------
. + /\_______/ +---+
. | / | .
. | 3+---+1 / | .
. N2 |--|RT2|-----+ 1| .
. | +---+ +---+ .
. | |RT3|----------------
. + +---+ .
. |2 .
. | .
. +------------+ .
. N4 .
..........................................
Figure 1: Area 1 with IP Addresses Shown
Network IPv6 prefix
-----------------------------------
N1 2001:0db8:c001:0200::/56
N2 2001:0db8:c001:0300::/56
N3 2001:0db8:c001:0100::/56
N4 2001:0db8:c001:0400::/56
Table 1: IPv6 Link Prefixes for Sample Network
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RFC 5340 OSPF for IPv6 July 2008
Router Interface Interface ID link-local address
-------------------------------------------------------
RT1 to N1 1 fe80:0001::RT1
to N3 2 fe80:0002::RT1
RT2 to N2 1 fe80:0001::RT2
to N3 2 fe80:0002::RT2
RT3 to N3 1 fe80:0001::RT3
to N4 2 fe80:0002::RT3
RT4 to N3 1 fe80:0001::RT4
Table 2: OSPF Interface IDs and Link-Local Addresses
Figure 1
4.4.3.1. LSA Options
The Options field in LSAs should be coded as follows. The V6-bit
should be set unless the router will not participate in transit IPv6
routing. The E-bit should be clear if and only if the attached area
is an OSPF stub or OSPF NSSA area. The E-bit should always be set in
AS scoped LSAs. The N-bit should be set if and only if the attached
area is an OSPF NSSA area. The R-bit should be set unless the router
will not participate in any transit routing. The DC-bit should be
set if and only if the router can correctly process the DoNotAge bit
when it appears in the LS age field of LSAs (see [DEMAND]). All
unrecognized bits in the Options field should be cleared.
The V6-bit and R-bit are only examined in Router-LSAs during the SPF
computation. In other LSA types containing options, they are set for
informational purposes only.
4.4.3.2. Router-LSAs
The LS type of a router-LSA is set to the value 0x2001. Router-LSAs
have area flooding scope. A router MAY originate one or more router-
LSAs for a given area. Each router-LSA contains an integral number
of interface descriptions. Taken together, the collection of router-
LSAs originated by the router for an area describes the collected
states of all the router's interfaces attached to the area. When
multiple router-LSAs are used, they are distinguished by their Link
State ID fields.
To the left of the Options field, the router capability bits V, E,
and B should be set according to Section 12.4.1 of [OSPFV2].
Each of the router's interfaces to the area is then described by
appending "link descriptions" to the router-LSA. Each link
description is 16 bytes long, consisting of five fields: (link) Type,
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RFC 5340 OSPF for IPv6 July 2008
Metric, Interface ID, Neighbor Interface ID, and Neighbor Router ID
(see Appendix A.4.3). Interfaces in the state "Down" or "Loopback"
are not described (although looped back interfaces can contribute
prefixes to intra-area-prefix-LSAs), nor are interfaces without any
full adjacencies described (except in the case of multiple Standby
Interfaces as described in Section 4.9). All other interfaces to the
area add zero, one, or more link descriptions. The number and
content of these depend on the interface type. Within each link
description, the Metric field is always set to the interface's output
cost, and the Interface ID field is set to the interface's OSPF
Interface ID.
Point-to-point interfaces
If the neighboring router is fully adjacent, add a Type 1 link
description (point-to-point). The Neighbor Interface ID field is
set to the Interface ID advertised by the neighbor in its Hello
packets, and the Neighbor Router ID field is set to the neighbor's
Router ID.
Broadcast and NBMA interfaces
If the router is fully adjacent to the link's Designated Router or
if the router itself is the Designated Router and is fully
adjacent to at least one other router, add a single Type 2 link
description (transit network). The Neighbor Interface ID field is
set to the Interface ID advertised by the Designated Router in its
Hello packets, and the Neighbor Router ID field is set to the
Designated Router's Router ID.
Virtual links
If the neighboring router is fully adjacent, add a Type 4 link
description (virtual). The Neighbor Interface ID field is set to
the Interface ID advertised by the neighbor in its Hello packets,
and the Neighbor Router ID field is set to the neighbor's Router
ID. Note that the output cost of a virtual link is calculated
during the routing table calculation (see Section 4.7).
Point-to-Multipoint interfaces
For each fully adjacent neighbor associated with the interface,
add a separate Type 1 link description (point-to-point) with the
Neighbor Interface ID field set to the Interface ID advertised by
the neighbor in its Hello packets and the Neighbor Router ID field
set to the neighbor's Router ID.
As an example, consider the router-LSA that router RT3 would
originate for Area 1 in Figure 1. Only a single interface must be
described, namely, that which connects to the transit network N3. It
assumes that RT4 has been elected the Designated Router of Network
N3.
Coltun, et al. Standards Track [Page 28]
RFC 5340 OSPF for IPv6 July 2008
; RT3's router-LSA for Area 1
LS age = 0 ;newly (re)originated
LS type = 0x2001 ;router-LSA
Link State ID = 0 ;first fragment
Advertising Router = 192.0.2.3 ;RT3's Router ID
bit E = 0 ;not an AS boundary router
bit B = 1 ;area border router
Options = (V6-bit|E-bit|R-bit)
Type = 2 ;connects to N3
Metric = 1 ;cost to N3
Interface ID = 1 ;RT3's Interface ID on N3
Neighbor Interface ID = 1 ;RT4's Interface ID on N3
Neighbor Router ID = 192.0.2.4 ; RT4's Router ID
RT3's router-LSA for Area 1
For example, if another router was added to Network N4, RT3 would
have to advertise a second link description for its connection to
(the now transit) network N4. This could be accomplished by
reoriginating the above router-LSA, this time with two link
descriptions. Or, a separate router-LSA could be originated with a
separate Link State ID (e.g., using a Link State ID of 1) to describe
the connection to N4.
Host routes for stub networks no longer appear in the router-LSA.
Rather, they are included in intra-area-prefix-LSAs.
4.4.3.3. Network-LSAs
The LS type of a network-LSA is set to the value 0x2002. Network-
LSAs have area flooding scope. A network-LSA is originated for every
broadcast or NBMA link with an elected Designated Router that is
fully adjacent with at least one other router on the link. The
network-LSA is originated by the link's Designated Router and lists
all routers on the link with which it is fully adjacent.
The procedure for originating network-LSAs in IPv6 is the same as the
IPv4 procedure documented in Section 12.4.2 of [OSPFV2], with the
following exceptions:
o An IPv6 network-LSA's Link State ID is set to the Interface ID of
the Designated Router on the link.
o IPv6 network-LSAs do not contain a Network Mask. All addressing
information formerly contained in the IPv4 network-LSA has now
been consigned to intra-Area-Prefix-LSAs originated by the link's
Designated Router.
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RFC 5340 OSPF for IPv6 July 2008
o The Options field in the network-LSA is set to the logical OR of
the Options fields contained within the link's associated link-
LSAs corresponding to fully adjacent neighbors. In this way, the
network link exhibits a capability when at least one fully
adjacent neighbor on the link requests that the capability be
advertised.
As an example, assuming that Router RT4 has been elected the
Designated Router of Network N3 in Figure 1, the following network-
LSA is originated:
; Network-LSA for Network N3
LS age = 0 ;newly (re)originated
LS type = 0x2002 ;network-LSA
Link State ID = 1 ;RT4's Interface ID on N3
Advertising Router = 192.0.2.4 ;RT4's Router ID
Options = (V6-bit|E-bit|R-bit)
Attached Router = 192.0.2.4 ;Router ID
Attached Router = 192.0.2.1 ;Router ID
Attached Router = 192.0.2.2 ;Router ID
Attached Router = 192.0.2.3 ;Router ID
Network-LSA for Network N3
4.4.3.4. Inter-Area-Prefix-LSAs
The LS type of an inter-area-prefix-LSA is set to the value 0x2003.
Inter-area-prefix-LSAs have area flooding scope. In IPv4, inter-
area-prefix-LSAs were called type 3 summary-LSAs. Each inter-area-
prefix-LSA describes a prefix external to the area, yet internal to
the Autonomous System.
The procedure for originating inter-area-prefix-LSAs in IPv6 is the
same as the IPv4 procedure documented in Sections 12.4.3 and 12.4.3.1
of [OSPFV2], with the following exceptions:
o The Link State ID of an inter-area-prefix-LSA has lost all of its
addressing semantics and simply serves to distinguish multiple
inter-area-prefix-LSAs that are originated by the same router.
o The prefix is described by the PrefixLength, PrefixOptions, and
Address Prefix fields embedded within the LSA body. Network Mask
is no longer specified.
o The NU-bit in the PrefixOptions field should be clear.
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RFC 5340 OSPF for IPv6 July 2008
o Link-local addresses MUST never be advertised in inter-area-
prefix-LSAs.
As an example, the following shows the inter-area-prefix-LSA that
Router RT4 originates into the OSPF backbone area, condensing all of
Area 1's prefixes into the single prefix 2001:0db8:c001::/48. The
cost is set to 4, which is the maximum cost of all of the individual
component prefixes. The prefix is padded out to an even number of
32-bit words, so that it consumes 64 bits of space instead of 48
bits.
; Inter-area-prefix-LSA for Area 1 addresses
; originated by Router RT4 into the backbone
LS age = 0 ;newly (re)originated
LS type = 0x2003 ;inter-area-prefix-LSA
Advertising Router = 192.0.2.4 ;RT4's ID
Metric = 4 ;maximum to components
PrefixLength = 48
PrefixOptions = 0
Address Prefix = 2001:0db8:c001 ;padded to 64-bits
Inter-area-prefix-LSA for Area 1 addresses originated
by Router
RT4 into the backbone
4.4.3.5. Inter-Area-Router-LSAs
The LS type of an inter-area-router-LSA is set to the value 0x2004.
Inter-area-router-LSAs have area flooding scope. In IPv4, inter-
area-router-LSAs were called type 4 summary-LSAs. Each inter-area-
router-LSA describes a path to a destination OSPF router (i.e., an AS
Boundary Router (ASBR)) that is external to the area yet internal to
the Autonomous System.
The procedure for originating inter-area-router-LSAs in IPv6 is the
same as the IPv4 procedure documented in Section 12.4.3 of [OSPFV2],
with the following exceptions:
o The Link State ID of an inter-area-router-LSA is no longer the
destination router's OSPF Router ID and now simply serves to
distinguish multiple inter-area-router-LSAs that are originated by
the same router. The destination router's Router ID is now found
in the body of the LSA.
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RFC 5340 OSPF for IPv6 July 2008
o The Options field in an inter-area-router-LSA should be set equal
to the Options field contained in the destination router's own
router-LSA. The Options field thus describes the capabilities
supported by the destination router.
As an example, consider the OSPF Autonomous System depicted in Figure
6 of [OSPFV2]. Router RT4 would originate into Area 1 the following
inter-area-router-LSA for destination router RT7.
; inter-area-router-LSA for AS boundary router RT7
; originated by Router RT4 into Area 1
LS age = 0 ;newly (re)originated
LS type = 0x2004 ;inter-area-router-LSA
Advertising Router = 192.0.2.4 ;RT4's ID
Options = (V6-bit|E-bit|R-bit) ;RT7's capabilities
Metric = 14 ;cost to RT7
Destination Router ID = Router RT7's ID
Inter-area-router-LSA for AS boundary router RT7 originated by Router
RT4 into Area 1
4.4.3.6. AS-External-LSAs
The LS type of an AS-external-LSA is set to the value 0x4005. AS-
external-LSAs have AS flooding scope. Each AS-external-LSA describes
a path to a prefix external to the Autonomous System.
The procedure for originating AS-external-LSAs in IPv6 is the same as
the IPv4 procedure documented in Section 12.4.4 of [OSPFV2], with the
following exceptions:
o The Link State ID of an AS-external-LSA has lost all of its
addressing semantics and simply serves to distinguish multiple AS-
external-LSAs that are originated by the same router.
o The prefix is described by the PrefixLength, PrefixOptions, and
Address Prefix fields embedded within the LSA body. Network Mask
is no longer specified.
o The NU-bit in the PrefixOptions field should be clear.
o Link-local addresses can never be advertised in AS-external-LSAs.
o The forwarding address is present in the AS-external-LSA if and
only if the AS-external-LSA's bit F is set.
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RFC 5340 OSPF for IPv6 July 2008
o The external route tag is present in the AS-external-LSA if and
only if the AS-external-LSA's bit T is set.
o The capability for an AS-external-LSA to reference another LSA has
been supported through the inclusion of the Referenced LS Type
field and the optional Referenced Link State ID field (the latter
present if and only if the Referenced LS Type is non-zero). This
capability is for future use; the Referenced LS Type should be set
to 0, and received non-zero values for this field should be
ignored until its use is defined.
As an example, consider the OSPF Autonomous System depicted in Figure
6 of [OSPFV2]. Assume that RT7 has learned its route to N12 via BGP
and that it wishes to advertise a Type 2 metric into the AS. Also
assume that the IPv6 prefix for N12 is the value 2001:0db8:0a00::/40.
RT7 would then originate the following AS-external-LSA for the
external network N12. Note that within the AS-external-LSA, N12's
prefix occupies 64 bits of space in order to maintain 32-bit
alignment.
; AS-external-LSA for Network N12,
; originated by Router RT7
LS age = 0 ;newly (re)originated
LS type = 0x4005 ;AS-external-LSA
Link State ID = 123 ;LSA type/scope unique identifier
Advertising Router = Router RT7's ID
bit E = 1 ;Type 2 metric
bit F = 0 ;no forwarding address
bit T = 1 ;external route tag included
Metric = 2
PrefixLength = 40
PrefixOptions = 0
Referenced LS Type = 0 ;no Referenced Link State ID
Address Prefix = 2001:0db8:0a00 ;padded to 64-bits
External Route Tag = as per BGP/OSPF interaction
AS-external-LSA for Network N12, originated by Router RT7
4.4.3.7. NSSA-LSAs
The LS type of an NSSA-LSA is set to the value 0x2007. NSSA-LSAs
have area flooding scope. Each NSSA-LSA describes a path to a prefix
external to the Autonomous System whose flooding scope is restricted
to a single NSSA area.
The procedure for originating NSSA-LSAs in IPv6 is the same as the
IPv4 procedure documented in [NSSA], with the following exceptions:
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RFC 5340 OSPF for IPv6 July 2008
o The Link State ID of an NSSA-LSA has lost all of its addressing
semantics and simply serves to distinguish multiple NSSA-LSAs that
are originated by the same router in the same area.
o The prefix is described by the PrefixLength, PrefixOptions, and
Address Prefix fields embedded within the LSA body. Network Mask
is no longer specified.
o The NU-bit in the PrefixOptions field should be clear.
o Link-local addresses can never be advertised in NSSA-LSAs.
o The forwarding address is present in the NSSA-LSA if and only if
the NSSA-LSA's bit F is set.
o The external route tag is present in the NSSA-LSA if and only if
the NSSA-LSA's bit T is set.
o The capability for an NSSA-LSA to reference another LSA has been
supported through the inclusion of the Referenced LS Type field
and the optional Referenced Link State ID field (the latter
present if and only if the Referenced LS Type is non-zero). This
capability is for future use; the Referenced LS Type should be set
to 0, and received non-zero values for this field should be
ignored until its use is defined.
An example of an NSSA-LSA would only differ from an AS-external-LSA
in that the LS type would be 0x2007 rather than 0x4005.
4.4.3.8. Link-LSAs
The LS type of a link-LSA is set to the value 0x0008. Link-LSAs have
link-local flooding scope. A router originates a separate link-LSA
for each attached link that supports two or more (including the
originating router itself) routers. Link-LSAs SHOULD NOT be
originated for virtual links.
Link-LSAs have three purposes:
1. They provide the router's link-local address to all other routers
attached to the link.
2. They inform other routers attached to the link of a list of IPv6
prefixes to associate with the link.
3. They allow the router to advertise a collection of Options bits
in the network-LSA originated by the Designated Router on a
broadcast or NBMA link.
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RFC 5340 OSPF for IPv6 July 2008
A link-LSA for a given Link L is built in the following fashion:
o The Link State ID is set to the router's Interface ID on Link L.
o The Router Priority of the router's interface to Link L is
inserted into the link-LSA.
o The link-LSA's Options field is set to reflect the router's
capabilities. On multi-access links, the Designated Router will
logically OR the link-LSA Options fields for all fully adjacent
neighbors in Link L's network-LSA.
o The router inserts its link-local address on Link L into the link-
LSA. This information will be used when the other routers on Link
L do their next-hop calculations (see Section 4.8.2).
o Each IPv6 address prefix that has been configured on Link L is
added to the link-LSA by specifying values for the PrefixLength,
PrefixOptions, and Address Prefix fields.
After building a link-LSA for a given link, the router installs the
link-LSA into the associated interface data structure and floods the
link-LSA on the link. All other routers on the link will receive the
link-LSA, but they will not flood the link-LSA on other links.
If LinkLSASuppression is configured for the interface and the
interface type is not broadcast or NBMA, origination of the link-LSA
may be suppressed. This implies that other routers on the link will
ascertain the router's next-hop address using a mechanism other than
the link-LSA (see Section 4.8.2). Refer to Appendix C.3 for a
description of the LinkLSASuppression interface configuration
parameter.
As an example, consider the link-LSA that RT3 will build for N3 in
Figure 1. Suppose that the prefix 2001:0db8:c001:0100::/56 has been
configured within RT3 for N3. This will result in the following
link-LSA that RT3 will flood only on N3. Note that not all routers
on N3 need be configured with the prefix; those not configured will
learn the prefix when receiving RT3's link-LSA.
Coltun, et al. Standards Track [Page 35]
RFC 5340 OSPF for IPv6 July 2008
; RT3's link-LSA for N3
LS age = 0 ;newly (re)originated
LS type = 0x0008 ;link-LSA
Link State ID = 1 ;RT3's Interface ID on N3
Advertising Router = 192.0.2.3 ;RT3's Router ID
Rtr Priority = 1 ;RT3's N3 Router Priority
Options = (V6-bit|E-bit|R-bit)
Link-local Interface Address = fe80:0001::RT3
# prefixes = 1
PrefixLength = 56
PrefixOptions = 0
Address Prefix = 2001:0db8:c001:0100 ;pad to 64-bits
RT3's link-LSA for N3
4.4.3.9. Intra-Area-Prefix-LSAs
The LS type of an intra-area-prefix-LSA is set to the value 0x2009.
Intra-area-prefix-LSAs have area flooding scope. An intra-area-
prefix-LSA has one of two functions. It either associates a list of
IPv6 address prefixes with a transit network link by referencing a
network-LSA, or associates a list of IPv6 address prefixes with a
router by referencing a router-LSA. A stub link's prefixes are
associated with its attached router.
A router MAY originate multiple intra-area-prefix-LSAs for a given
area. Each intra-area-prefix-LSA has a unique Link State ID and
contains an integral number of prefix descriptions.
A link's Designated Router originates one or more intra-area-prefix-
LSAs to advertise the link's prefixes throughout the area. For a
link L, L's Designated Router builds an intra-area-prefix-LSA in the
following fashion:
o In order to indicate that the prefixes are to be associated with
the Link L, the fields Referenced LS Type, Referenced Link State
ID, and Referenced Advertising Router are set to the corresponding
fields in Link L's network-LSA (namely, LS type, Link State ID,
and Advertising Router respectively). This means that the
Referenced LS Type is set to 0x2002, the Referenced Link State ID
is set to the Designated Router's Interface ID on Link L, and the
Referenced Advertising Router is set to the Designated Router's
Router ID.
o Each link-LSA associated with Link L is examined (these are in the
Designated Router's interface structure for Link L). If the link-
LSA's Advertising Router is fully adjacent to the Designated
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RFC 5340 OSPF for IPv6 July 2008
Router and the Link State ID matches the neighbor's interface ID,
the list of prefixes in the link-LSA is copied into the intra-
area-prefix-LSA that is being built. Prefixes having the NU-bit
and/or LA-bit set in their Options field SHOULD NOT be copied, nor
should link-local addresses be copied. Each prefix is described
by the PrefixLength, PrefixOptions, and Address Prefix fields.
Multiple prefixes having the same PrefixLength and Address Prefix
are considered to be duplicates. In this case, their
PrefixOptions fields should be logically OR'ed together, and a
single instance of the duplicate prefix should be included in the
intra-area-prefix-LSA. The Metric field for all prefixes is set
to 0.
o The "# prefixes" field is set to the number of prefixes that the
router has copied into the LSA. If necessary, the list of
prefixes can be spread across multiple intra-area-prefix-LSAs in
order to keep the LSA size small.
A router builds an intra-area-prefix-LSA to advertise prefixes for
its attached stub links, looped-back interfaces, and hosts. A Router
RTX would build its intra-area-prefix-LSA in the following fashion:
o In order to indicate that the prefixes are to be associated with
the Router RTX itself, RTX sets the Referenced LS Type to 0x2001,
the Referenced Link State ID to 0, and the Referenced Advertising
Router to RTX's own Router ID.
o Router RTX examines its list of interfaces to the area. If the
interface is in the state Down, its prefixes are not included. If
the interface has been reported in RTX's router-LSA as a Type 2
link description (link to transit network), prefixes that will be
included in the intra-area-prefix-LSA for the link are skipped.
However, any prefixes that would normally have the LA-bit set
SHOULD be advertised independent of whether or not the interface
is advertised as a transit link. If the interface type is point-
to-multipoint or the interface is in the state Loopback, the
global scope IPv6 addresses associated with the interface (if any)
are copied into the intra-area-prefix-LSA with the PrefixOptions
LA-bit set, the PrefixLength set to 128, and the metric set to 0.
Otherwise, the list of global prefixes configured in RTX for the
link are copied into the intra-area-prefix-LSA by specifying the
PrefixLength, PrefixOptions, and Address Prefix fields. The
Metric field for each of these prefixes is set to the interface's
output cost.
o RTX adds the IPv6 prefixes for any directly attached hosts
belonging to the area (see Appendix C.7) to the intra-area-prefix-
LSA.
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o If RTX has one or more virtual links configured through the area,
it includes one of its global scope IPv6 interface addresses in
the LSA (if it hasn't already), setting the LA-bit in the
PrefixOptions field, the PrefixLength to 128, and the Metric to 0.
This information will be used later in the routing calculation so
that the two ends of the virtual link can discover each other's
IPv6 addresses.
o The "# prefixes" field is set to the number of prefixes that the
router has copied into the LSA. If necessary, the list of
prefixes can be spread across multiple intra-area-prefix-LSAs in
order to keep the LSA size small.
For example, the intra-area-prefix-LSA originated by RT4 for Network
N3 (assuming that RT4 is N3's Designated Router) and the intra-area-
prefix-LSA originated into Area 1 by Router RT3 for its own prefixes
are pictured below.
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; RT4's Intra-area-prefix-LSA for network link N3
LS age = 0 ;newly (re)originated
LS type = 0x2009 ;Intra-area-prefix-LSA
Link State ID = 5 ;LSA type/scope unique identifier
Advertising Router = 192.0.2.4 ;RT4's Router ID
# prefixes = 1
Referenced LS Type = 0x2002 ;network-LSA reference
Referenced Link State ID = 1
Referenced Advertising Router = 192.0.2.4
PrefixLength = 56 ;N3's prefix
PrefixOptions = 0
Metric = 0
Address Prefix = 2001:0db8:c001:0100 ;pad
; RT3's Intra-area-prefix-LSA for its own prefixes
LS age = 0 ;newly (re)originated
LS type = 0x2009 ;Intra-area-prefix-LSA
Link State ID = 177 ;LSA type/scope unique identifier
Advertising Router = 192.0.2.3 ;RT3's Router ID
# prefixes = 1
Referenced LS Type = 0x2001 ;router-LSA reference
Referenced Link State ID = 0
Referenced Advertising Router = 192.0.2.3
PrefixLength = 56 ;N4's prefix
PrefixOptions = 0
Metric = 2 ;N4 interface cost
Address Prefix = 2001:0db8:c001:0400 ;pad
Intra-area-prefix-LSA for Network Link N3
When network conditions change, it may be necessary for a router to
move prefixes from one intra-area-prefix-LSA to another. For
example, if the router is the Designated Router for a link but the
link has no other attached routers, the link's prefixes are
advertised in an intra-area-prefix-LSA referring to the Designated
Router's router-LSA. When additional routers appear on the link, a
network-LSA is originated for the link and the link's prefixes are
moved to an intra-area-prefix-LSA referring to the network-LSA.
Note that in the intra-area-prefix-LSA, the Referenced Advertising
Router is always equal to the router that is originating the intra-
area-prefix-LSA (i.e., the LSA's Advertising Router). The reason the
Referenced Advertising Router field appears is that, even though it
is currently redundant, it may not be in the future. We may sometime
want to use the same LSA format to advertise address prefixes for
other protocol suites. In this case, the Designated Router may not
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be running the other protocol suite, and so another of the link's
routers may need to originate the intra-area-prefix-LSA. In that
case, the Referenced Advertising Router and Advertising Router would
be different.
4.4.4. Future LSA Validation
It is expected that new LSAs will be defined that will not be
processed during the Shortest Path First (SPF) calculation as
described in Section 4.8, for example, OSPFv3 LSAs corresponding to
information advertised in OSPFv2 using opaque LSAs [OPAQUE]. In
general, the new information advertised in future LSAs should not be
used unless the OSPFv3 router originating the LSA is reachable.
However, depending on the application and the data advertised, this
reachability validation MAY be done less frequently than every SPF
calculation.
To facilitate inter-area reachability validation, any OSPFv3 router
originating AS scoped LSAs is considered an AS Boundary Router
(ASBR).
4.5. Flooding
Most of the flooding algorithm remains unchanged from the IPv4
flooding mechanisms described in Section 13 of [OSPFV2]. In
particular, the protocol processes for determining which LSA instance
is newer (Section 13.1 of [OSPFV2]), responding to updates of self-
originated LSAs (Section 13.4 of [OSPFV2]), sending Link State
Acknowledgment packets (Section 13.5 of [OSPFV2]), retransmitting
LSAs (Section 13.6 of [OSPFV2]), and receiving Link State
Acknowledgment packets (Section 13.7 of [OSPFV2]), are exactly the
same for IPv6 and IPv4.
However, the addition of flooding scope and unknown LSA type handling
(see Appendix A.4.2.1) has caused some changes in the OSPF flooding
algorithm: the reception of Link State Updates (Section 13 in
[OSPFV2]) and the sending of Link State Updates (Section 13.3 of
[OSPFV2]) must take into account the LSA's scope and U-bit setting.
Also, installation of LSAs into the OSPF database (Section 13.2 of
[OSPFV2]) causes different events in IPv6, due to the reorganization
of LSA types and the IPv6 LSA contents. These changes are described
in detail below.
4.5.1. Receiving Link State Update Packets
The encoding of flooding scope in the LS type and the need to process
unknown LS types cause modifications to the processing of received
Link State Update packets. As in IPv4, each LSA in a received Link
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State Update packet is examined. In IPv4, eight steps are executed
for each LSA, as described in Section 13 of [OSPFV2]. For IPv6, all
the steps are the same, except that Steps 2 and 3 are modified as
follows:
(2) Examine the LSA's LS type. Discard the LSA and get
the next one from the Link State Update packet if the
interface area has been configured as a stub or
NSSA area and the LS type indicates "AS flooding scope".
This generalizes the IPv4 behavior where AS-external-LSAs
and AS-scoped opaque LSAs [OPAQUE] are not flooded
throughout stub or NSSA areas.
(3) Else if the flooding scope in the LSA's LS type is set to
"reserved", discard the LSA and get the next one from
the Link State Update packet.
Steps 5b (sending Link State Update packets) and 5d (installing LSAs
in the link-state database) in Section 13 of [OSPFV2] are also
somewhat different for IPv6, as described in Sections 4.5.2 and 4.5.3
below.
4.5.2. Sending Link State Update Packets
The sending of Link State Update packets is described in Section 13.3
of [OSPFV2]. For IPv4 and IPv6, the steps for sending a Link State
Update packet are the same (steps 1 through 5 of Section 13.3 in
[OSPFV2]). However, the list of eligible interfaces on which to
flood the LSA is different. For IPv6, the eligible interfaces are
selected based on the following factors:
o The LSA's flooding scope.
o For LSAs with area or link-local flooding scope, the particular
area or interface with which the LSA is associated.
o Whether the LSA has a recognized LS type.
o The setting of the U-bit in the LS type. If the U-bit is set to
0, unrecognized LS types are treated as having link-local scope.
If set to 1, unrecognized LS types are stored and flooded as if
they were recognized.
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Choosing the set of eligible interfaces then breaks into the
following cases:
Case 1
The LSA's LS type is recognized. In this case, the set of
eligible interfaces is set depending on the flooding scope encoded
in the LS type. If the flooding scope is "AS flooding scope", the
eligible interfaces are all router interfaces excepting virtual
links. In addition, AS-external-LSAs are not flooded on
interfaces connecting to stub or NSSA areas. If the flooding
scope is "area flooding scope", the eligible interfaces are those
interfaces connecting to the LSA's associated area. If the
flooding scope is "link-local flooding scope", then there is a
single eligible interface, the one connecting to the LSA's
associated link (which is also the interface on which the LSA was
received in a Link State Update packet).
Case 2
The LS type is unrecognized and the U-bit in the LS type is set to
0 (treat the LSA as if it had link-local flooding scope). In this
case, there is a single eligible interface, namely, the interface
on which the LSA was received.
Case 3
The LS type is unrecognized, and the U-bit in the LS type is set
to 1 (store and flood the LSA as if the type is understood). In
this case, select the eligible interfaces based on the encoded
flooding scope the same as in Case 1 above.
A further decision must sometimes be made before adding an LSA to a
given neighbor's link-state retransmission list (Step 1d in Section
13.3 of [OSPFV2]). If the LS type is recognized by the router but
not by the neighbor (as can be determined by examining the Options
field that the neighbor advertised in its Database Description
packet) and the LSA's U-bit is set to 0, then the LSA should be added
to the neighbor's link-state retransmission list if and only if that
neighbor is the Designated Router or Backup Designated Router for the
attached link. The LS types described in detail by this document,
namely, router-LSAs (LS type 0x2001), network-LSAs (0x2002), inter-
area-prefix-LSAs (0x2003), inter-area-router-LSAs (0x2004), NSSA-LSAs
(0x2007), AS-external-LSAs (0x4005), link-LSAs (0x0008), and Intra-
Area-Prefix-LSAs (0x2009), are assumed to be understood by all
routers. However, all LS types MAY not be understood by all routers.
For example, a new LSA type with its U-bit set to 0 MAY only be
understood by a subset of routers. This new LS type should only be
flooded to an OSPF neighbor that understands the LS type or when the
neighbor is the Designated Router or Backup Designated Router for the
attached link.
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The previous paragraph solves a problem for IPv4 OSPF extensions,
which require that the Designated Router support the extension in
order to have the new LSA types flooded across broadcast and NBMA
networks.
4.5.3. Installing LSAs in the Database
There are three separate places to store LSAs, depending on their
flooding scope. LSAs with AS flooding scope are stored in the global
OSPF data structure (see Section 4.1) as long as their LS type is
known or their U-bit is 1. LSAs with area flooding scope are stored
in the appropriate area data structure (see Section 4.1.1) as long as
their LS type is known or their U-bit is 1. LSAs with link-local
flooding scope, and those LSAs with unknown LS type and U-bit set to
0 (treat the LSA as if it had link-local flooding scope), are stored
in the appropriate interface data structure.
When storing the LSA into the link-state database, a check must be
made to see whether the LSA's contents have changed. Changes in
contents are indicated exactly as in Section 13.2 of [OSPFV2]. When
an LSA's contents have been changed, the following parts of the
routing table must be recalculated, based on the LSA's LS type:
Router-LSAs, Network-LSAs, Intra-Area-Prefix-LSAs, and Link-LSAs
The entire routing table is recalculated, starting with the
shortest-path calculation for each area (see Section 4.8).
Inter-Area-Prefix-LSAs and Inter-Area-Router-LSAs
The best route to the destination described by the LSA must be
recalculated (see Section 16.5 in [OSPFV2]). If this destination
is an AS boundary router, it may also be necessary to re-examine
all the AS-external-LSAs.
AS-external-LSAs and NSSA-LSAs
The best route to the destination described by the AS-external-LSA
or NSSA-LSA must be recalculated (see Section 16.6 in [OSPFV2] and
Section 2.0 in [NSSA]).
As in IPv4, any old instance of the LSA must be removed from the
database when the new LSA is installed. This old instance must also
be removed from all neighbors' link-state retransmission lists.
4.6. Definition of Self-Originated LSAs
In IPv6, the definition of a self-originated LSA has been simplified
from the IPv4 definition appearing in Sections 13.4 and 14.1 of
[OSPFV2]. For IPv6, self-originated LSAs are those LSAs whose
Advertising Router is equal to the router's own Router ID.
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4.7. Virtual Links
OSPF virtual links for IPv4 are described in Section 15 of [OSPFV2].
Virtual links are the same in IPv6, with the following exceptions:
o LSAs having AS flooding scope are never flooded over virtual
adjacencies, nor are LSAs with AS flooding scope summarized over
virtual adjacencies during the database exchange process. This is
a generalization of the IPv4 treatment of AS-external-LSAs.
o The IPv6 interface address of a virtual link MUST be an IPv6
address having global scope, instead of the link-local addresses
used by other interface types. This address is used as the IPv6
source for OSPF protocol packets sent over the virtual link.
Hence, a link-LSA SHOULD NOT be originated for a virtual link
since the virtual link has no link-local address or associated
prefixes.
o Likewise, the virtual neighbor's IPv6 address is an IPv6 address
with global scope. To enable the discovery of a virtual
neighbor's IPv6 address during the routing calculation, the
neighbor advertises its virtual link's IPv6 interface address in
an intra-area-prefix-LSA originated for the virtual link's transit
area (see Section 4.4.3.9 and Section 4.8.1).
o Like all other IPv6 OSPF interfaces, virtual links are assigned
unique (within the router) Interface IDs. These are advertised in
Hellos sent over the virtual link and in the router's router-LSAs.
4.8. Routing Table Calculation
The IPv6 OSPF routing calculation proceeds along the same lines as
the IPv4 OSPF routing calculation, following the five steps specified
by Section 16 of [OSPFV2]. High-level differences between the IPv6
and IPv4 calculations include:
o Prefix information has been removed from router-LSAs and network-
LSAs and is now advertised in intra-area-prefix-LSAs. Whenever
[OSPFV2] specifies that stub networks within router-LSAs be
examined, IPv6 will instead examine prefixes within intra-area-
prefix-LSAs.
o Type 3 and 4 summary-LSAs have been renamed inter-area-prefix-LSAs
and inter-area-router-LSAs respectively.
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o Addressing information is no longer encoded in Link State IDs and
is now only found within the body of LSAs.
o In IPv6, a router can originate multiple router-LSAs,
distinguished by Link State ID, within a single area. These
router-LSAs MUST be treated as a single aggregate by the area's
shortest-path calculation (see Section 4.8.1).
For each area, the shortest-path tree calculation creates routing
table entries for the area's routers and transit links (see
Section 4.8.1). These entries are then used when processing intra-
area-prefix-LSAs, inter-area-prefix-LSAs, and inter-area-router-LSAs,
as described in Section 4.8.3.
Events generated as a result of routing table changes (Section 16.7
of [OSPFV2]) and the equal-cost multipath logic (Section 16.8 of
[OSPFV2]) are identical for both IPv4 and IPv6.
4.8.1. Calculating the Shortest-Path Tree for an Area
The IPv4 shortest-path calculation is contained in Section 16.1 of
[OSPFV2]. The graph used by the shortest-path tree calculation is
identical for both IPv4 and IPv6. The graph's vertices are routers
and transit links, represented by router-LSAs and network-LSAs
respectively. A router is identified by its OSPF Router ID, while a
transit link is identified by its Designated Router's Interface ID
and OSPF Router ID. Both routers and transit links have associated
routing table entries within the area (see Section 4.3).
Section 16.1 of [OSPFV2] splits up the shortest-path calculations
into two stages. First, the Dijkstra calculation is performed, and
then the stub links are added onto the tree as leaves. The IPv6
calculation maintains this split.
The Dijkstra calculation for IPv6 is identical to that specified for
IPv4, with the following exceptions (referencing the steps from the
Dijkstra calculation as described in Section 16.1 of [OSPFV2]):
o The Vertex ID for a router is the OSPF Router ID. The Vertex ID
for a transit network is a combination of the Interface ID and
OSPF Router ID of the network's Designated Router.
o In Step 2, when a router Vertex V has just been added to the
shortest-path tree, there may be multiple LSAs associated with the
router. All router-LSAs with the Advertising Router set to V's
OSPF Router ID MUST be processed as an aggregate, treating them as
fragments of a single large router-LSA. The Options field and the
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router type bits (bits Nt, V, E, and B) should always be taken
from the router-LSA with the smallest Link State ID.
o Step 2a is not needed in IPv6, as there are no longer stub network
links in router-LSAs.
o In Step 2b, if W is a router and the router-LSA V6-bit or R-bit is
not set in the LSA options, the transit link W is ignored and V's
next link is examined.
o In Step 2b, if W is a router, there may again be multiple LSAs
associated with the router. All router-LSAs with the Advertising
Router set to W's OSPF Router ID MUST be processed as an
aggregate, treating them as fragments of a single large router-
LSA.
o In Step 4, there are now per-area routing table entries for each
of an area's routers rather than just the area border routers.
These entries subsume all the functionality of IPv4's area border
router routing table entries, including the maintenance of virtual
links. When the router added to the area routing table in this
step is the other end of a virtual link, the virtual neighbor's IP
address is set as follows: The collection of intra-area-prefix-
LSAs originated by the virtual neighbor is examined, with the
virtual neighbor's IP address being set to the first prefix
encountered with the LA-bit set.
o Routing table entries for transit networks, which are no longer
associated with IP networks, are also calculated in Step 4 and
added to the per-area routing table.
The next stage of the shortest-path calculation proceeds similarly to
the two steps of the second stage of Section 16.1 in [OSPFV2].
However, instead of examining the stub links within router-LSAs, the
list of the area's intra-area-prefix-LSAs is examined. A prefix
advertisement whose NU-bit is set SHOULD NOT be included in the
routing calculation. The cost of any advertised prefix is the sum of
the prefix's advertised metric plus the cost to the transit vertex
(either router or transit network) identified by intra-area-prefix-
LSA's Referenced LS Type, Referenced Link State ID, and Referenced
Advertising Router fields. This latter cost is stored in the transit
vertex's routing table entry for the area.
This specification does not require that the above algorithm be used
to calculate the intra-area shortest-path tree. However, if another
algorithm or optimization is used, an identical shortest-path tree
must be produced. It is also important that any alternate algorithm
or optimization maintain the requirement that transit vertices must
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be bidirectional for inclusion in the tree. Alternate algorithms and
optimizations are beyond the scope of this specification.
4.8.2. The Next-Hop Calculation
In IPv6, the calculation of the next-hop's IPv6 address (which will
be a link-local address) proceeds along the same lines as the IPv4
next-hop calculation (see Section 16.1.1 of [OSPFV2]). However,
there are some differences. When calculating the next-hop IPv6
address for a router (call it Router X) that shares a link with the
calculating router, the calculating router assigns the next-hop IPv6
address to be the link-local interface address contained in Router
X's link-LSA (see Appendix A.4.9) for the link. This procedure is
necessary for some link types, for example NBMA, where the two
routers need not be neighbors and might not be exchanging OSPF Hello
packets. For other link types, the next-hop address may be
determined via the IPv6 source address in the neighbor's Hello
packet.
Additionally, when calculating routes for the area's intra-area-
prefix-LSAs, the parent vertex can be either a router-LSA or network-
LSA. This is in contrast to the second stage of the OSPFv2 intra-
area SPF (Section 16.1 in [OSPFV2]) where the parent vertex is always
a router-LSA. In the case where the intra-area-prefix-LSA's
referenced LSA is a directly connected network-LSA, the prefixes are
also considered to be directly connected. In this case, the next hop
is solely the outgoing link and no IPv6 next-hop address is selected.
4.8.3. Calculating the Inter-Area Routes
Calculation of inter-area routes for IPv6 proceeds along the same
lines as the IPv4 calculation in Section 16.2 of [OSPFV2], with the
following modifications:
o The names of the Type 3 summary-LSAs and Type 4 summary-LSAs have
been changed to inter-area-prefix-LSAs and inter-area-router-LSAs
respectively.
o The Link State ID of the above LSA types no longer encodes the
network or router described by the LSA. Instead, an address
prefix is contained in the body of an inter-area-prefix-LSA and an
advertised AS boundary router's OSPF Router ID is carried in the
body of an inter-area-router-LSA.
o Prefixes having the NU-bit set in their PrefixOptions field should
be ignored by the inter-area route calculation.
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When a single inter-area-prefix-LSA or inter-area-router-LSA has
changed, the incremental calculations outlined in Section 16.5 of
[OSPFV2] can be performed instead of recalculating the entire routing
table.
4.8.4. Examining Transit Areas' Summary-LSAs
Examination of transit areas' summary-LSAs in IPv6 proceeds along the
same lines as the IPv4 calculation in Section 16.3 of [OSPFV2],
modified in the same way as the IPv6 inter-area route calculation in
Section 4.8.3.
4.8.5. Calculating AS External and NSSA Routes
The IPv6 AS external route calculation proceeds along the same lines
as the IPv4 calculation in Section 16.4 of [OSPFV2] and Section 2.5
of [NSSA], with the following exceptions:
o The Link State ID of the AS-external-LSA and NSSA-LSA types no
longer encodes the network described by the LSA. Instead, an
address prefix is contained in the body of the LSA.
o The default route in AS-external-LSAs or NSSA-LSAs is advertised
by a zero-length prefix.
o Instead of comparing the AS-external-LSA's or NSSA-LSA's
Forwarding Address field to 0.0.0.0 to see whether a forwarding
address has been used, the bit F in the respective LSA is
examined. A forwarding address is in use if and only if bit F is
set.
o Prefixes having the NU-bit set in their PrefixOptions field should
be ignored by the inter-area route calculation.
o AS Boundary Router (ASBR) and forwarding address selection will
proceed the same as if RFC1583Compatibility is disabled.
Furthermore, RFC1583Compatibility is not an OSPF for IPv6
configuration parameter. Refer to Appendix C.1.
When a single AS-external-LSA or NSSA-LSA has changed, the
incremental calculations outlined in Section 16.6 of [OSPFV2] can be
performed instead of recalculating the entire routing table.
4.9. Multiple Interfaces to a Single Link
In OSPF for IPv6, a router may have multiple interfaces to a single
link associated with the same OSPF instance and area. All interfaces
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will be used for the reception and transmission of data traffic while
only a single interface sends and receives OSPF control traffic. In
more detail:
o Each of the multiple interfaces is assigned a different Interface
ID. A router will automatically detect that multiple interfaces
are attached to the same link when a Hello packet is received with
one of the router's link-local addresses as the source address and
an Interface ID other than the Interface ID of the receiving
interface.
o Each of the multiple interfaces MUST be configured with the same
Interface Instance ID to be considered on the same link. If an
interface has multiple Instance IDs, it will be grouped with other
interfaces based on matching Instance IDs. Each Instance ID will
be treated uniquely with respect to groupings of multiple
interfaces on the same link. For example, if interface A is
configured with Instance IDs 1 and 35, and interface B is
configured with Instance ID 35, interface B may be the Active
Interface for Instance ID 35 but interface A will be active for
Instance ID 1.
o The router will ignore OSPF packets other than Hello packets on
all but one of the interfaces attached to the link. It will only
send its OSPF control packets (including Hello packets) on a
single interface. This interface is designated the Active
Interface and other interfaces attached to the same link will be
designated Standby Interfaces. The choice of the Active Interface
is implementation dependent. For example, the interface with the
highest Interface ID could be chosen. If the router is elected
Designated Router, it will be the Active Interface's Interface ID
that will be used as the network-LSA's Link State ID.
o All of the interfaces to the link (Active and Standby) will appear
in the router-LSA. In addition, a link-LSA will be generated for
each of the interfaces. In this way, all interfaces will be
included in OSPF's routing calculations.
o Any link-local scope LSAs that are originated for a Standby
Interface will be flooded over the Active Interface.
If a Standby Interface goes down, then the link-local scope LSAs
originated for the Standby Interfaces MUST be flushed on the
Active Interface.
o Prefixes on Standby Interfaces will be processed the same way as
prefixes on the Active Interface. For example, if the router is
the DR for the link, the Active Interface's prefixes are included
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in an intra-area-prefix-LSA which is associated with the Active
Interface's network-LSA; prefixes from Standby Interfaces on the
link will also be included in that intra-area-prefix LSA.
Similarly, if the link is a stub link, then the prefixes for the
Active and Standby Interfaces will all be included in the same
intra-area-prefix-LSA that is associated with the router-LSA.
o If the Active Interface fails, a new Active Interface will have to
take over. The new Active Interface SHOULD form all new neighbor
adjacencies with routers on the link. This failure can be
detected when the router's other interfaces to the Active
Interface's link cease to hear the router's Hellos or through
internal mechanisms, e.g., monitoring the Active Interface's
status.
o If the network becomes partitioned with different local interfaces
attaching to different network partitions, multiple interfaces
will become Active Interfaces and function independently.
o During the SPF calculation when a network-LSA for a network that
is directly connected to the root vertex is being examined, all of
the multiple interfaces to the link of adjacent router-LSAs must
be used in the next-hop calculation.
This can be accomplished during the back link check (see Section
16.1, Step 2 (B), in [OSPFV2]) by examining each link of the
router-LSA and making a list of the links that point to the
network-LSA. The Interface IDs for links in this list are then
used to find the corresponding link-LSAs and the link-local
addresses used as next hops when installing equal-cost paths in
the routing table.
o The interface state machine is modified to add the state Standby.
See Section 4.9.1 for a description of the Standby state.
4.9.1. Standby Interface State
In this state, the interface is one of multiple interfaces to a link
and this interface is designated Standby and is not sending or
receiving control packets. The interface will continue to receive
the Hello packets sent by the Active Interface. The interface will
maintain a timer, the Active Interface Timer, with the same interval
as the RouterDeadInterval. This timer will be reset whenever an OSPF
Hello packet is received from the Active Interface to the link.
Two new events are added to the list of events that cause interface
state changes: MultipleInterfacesToLink and ActiveInterfaceDead. The
descriptions of these events are as follows:
Coltun, et al. Standards Track [Page 50]
RFC 5340 OSPF for IPv6 July 2008
MultipleInterfacesToLink
An interfaces on the router has received a Hello packet from
another interface on the same router. One of the interfaces is
designated as the Active Interface and the other interface is
designated as a Standby Interface. The Standby Interface
transitions to the Standby state.
ActiveInterfaceDead
There has been an indication that a Standby Interface is no longer
on a link with an Active Interface. The firing of the Active
Interface Timer is one indication of this event, as it indicates
that the Standby Interface has not received an OSPF Hello packet
from the Active Interface for the RouterDeadInterval. Other
indications may come from internal notifications, such as the
Active Interface being disabled through a configuration change.
Any indication internal to the router, such that the router knows
the Active Interface is no longer active on the link, can trigger
the ActiveInterfaceDead event for a Standby Interface.
Interface state machine additions include:
State(s): Waiting, DR Other, Backup, or DR
Event: MultipleInterfacesToLink
New state: Standby
Action: All interface variables are reset and interface
timers disabled. Also, all neighbor connections
associated with the interface are destroyed. This
is done by generating the event KillNbr on all
associated neighbors. The Active Interface Timer is
started and the interface will listen for OSPF Hello
packets from the link's Active Interface.
State(s): Standby
Event: ActiveInterfaceDead
New state: Down
Action: The Active Interface Timer is first disabled. Then
the InterfaceUp event is invoked.
Standby Interface State Machine Additions
Coltun, et al. Standards Track [Page 51]
RFC 5340 OSPF for IPv6 July 2008
5. Security Considerations
When running over IPv6, OSPFv3 relies on the IP Authentication Header
(see [IPAUTH]) and the IP Encapsulating Security Payload (see
[IPESP]) to ensure integrity and authentication/confidentiality of
protocol packets. This is described in [OSPFV3-AUTH].
Most OSPFv3 implementations will be running on systems that support
multiple protocols with their own independent security assumptions
and domains. When IPsec is used to protect OSPFv3 packets, it is
important for the implementation to check the IPsec Security
Association (SA) and local SA database to ensure the OSPF packet
originated from a source that is trusted for OSPFv3. This is
required to eliminate the possibility that the packet was
authenticated using an SA defined for another protocol running on the
same system.
The mechanisms in [OSPFV3-AUTH] do not provide protection against
compromised, malfunctioning, or misconfigured routers. Such routers
can, either accidentally or deliberately, cause malfunctions
affecting the whole routing domain. The reader is encouraged to
consult [GENERIC-THREATS] for a more comprehensive description of
threats to routing protocols.
6. Manageability Considerations
The Management Information Base (MIB) for OSPFv3 is defined in
[OSPFV3-MIB].
7. IANA Considerations
Most OSPF for IPv6 IANA considerations are documented in [OSPF-IANA].
IANA has updated the reference for RFC 2740 to this document.
Additionally, this document introduces the following IANA
requirements that were not present in [OSPFV3]:
o Reserves the options with the values 0x000040 and 0x000080 for
migrated OSPFv2 options in the OSPFv3 Options registry defined in
[OSPF-IANA]. For information on the OSPFv3 Options field, refer
to Appendix A.2.
o Adds the prefix option P-bit with value 0x08 to the OSPFv3 Prefix
Options registry defined in [OSPF-IANA]. For information on
OSPFv3 Prefix Options, refer to Appendix A.4.1.1.
Coltun, et al. Standards Track [Page 52]
RFC 5340 OSPF for IPv6 July 2008
o Adds the prefix option DN-bit with value 0x10 to the OSPFv3 Prefix
Options registry defined in [OSPF-IANA]. For information on
OSPFv3 Prefix Options, refer to Appendix A.4.1.1.
7.1. MOSPF for OSPFv3 Deprecation IANA Considerations
With the deprecation of MOSPF for OSPFv3, the following code points
are available for reassignment. Refer to [OSPF-IANA] for information
on the respective registries. This document:
o Deprecates the MC-bit with value 0x000004 in the OSPFv3 Options
registry.
o Deprecates Group-membership-LSA with value 6 in OSPFv3 LSA
Function Code registry.
o Deprecates MC-bit with value 0x04 in the OSPFv3 Prefix Options
registry.
The W-bit in the OSPFv3 Router Properties has also been deprecated.
This requires a new registry for OSPFv3 router properties since it
will diverge from the OSPFv2 Router Properties.
Registry Name: OSPFv3 Router Properties Registry
Reference: RFC 5340
Registration Procedures: Standards Action
Registry:
Value Description Reference
------ ------------- ---------
0x01 B-bit RFC 5340
0x02 E-bit RFC 5340
0x04 V-bit RFC 5340
0x08 Deprecated RFC 5340
0x10 Nt-bit RFC 5340
OSPFv3 Router Properties Registry
8. Acknowledgments
The RFC text was produced using Marshall Rose's xml2rfc tool.
The following individuals contributed comments that were incorporated
into this document:
o Harold Rabbie for his description of protocol details that needed
to be clarified for OSPFv3 NSSA support.
Coltun, et al. Standards Track [Page 53]
RFC 5340 OSPF for IPv6 July 2008
o Nic Neate for his pointing out that there needed to be changes for
unknown LSA types handling in the processing of Database
Description packets.
o Jacek Kwiatkowski for being the first to point out that the V6-
and R-bits are not taken into account in the OSPFv3 intra-area SPF
calculation.
o Michael Barnes recognized that the support for multiple interfaces
to a single link was broken (see Section 4.9) and provided the
description of the current protocol mechanisms. Abhay Roy
reviewed and suggested improvements to the mechanisms.
o Alan Davey reviewed and commented on document revisions.
o Vivek Dubey reviewed and commented on document revisions.
o Manoj Goyal and Vivek Dubey complained enough about link-LSAs
being unnecessary to compel introduction of the LinkLSASuppression
interface configuration parameter.
o Manoj Goyal for pointing out that the next-hop calculation for
intra-area-prefix-LSAs corresponding to network vertices was
unclear.
o Ramana Koppula reviewed and commented on document revisions.
o Paul Wells reviewed and commented on document revisions.
o Amir Khan reviewed and commented on document revisions.
o Dow Street and Wayne Wheeler commented on the addition of the DN-
bit to OSPFv3.
o Mitchell Erblichs provided numerous editorial comments.
o Russ White provided numerous editorial comments.
o Kashima Hiroaki provided editorial comments.
o Sina Mirtorabi suggested that OSPFv3 should be aligned with OSPFv2
with respect to precedence and should map it to IPv6 traffic class
as specified in RFC 2474. Steve Blake helped with the text.
o Faraz Shamin reviewed a late version of the document and provided
editorial comments.
Coltun, et al. Standards Track [Page 54]
RFC 5340 OSPF for IPv6 July 2008
o Christian Vogt performed the General Area Review Team (Gen-ART)
review and provided comments.
o Dave Ward, Dan Romascanu, Tim Polk, Ron Bonica, Pasi Eronen, and
Lars Eggert provided comments during the IESG review. Also,
thanks to Pasi for the text in Section 5 relating to routing
threats.
9. References
9.1. Normative References
[DEMAND] Moy, J., "Extending OSPF to Support Demand
Circuits", RFC 1793, April 1995.
[DIFF-SERV] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field
(DS Field) in the IPv4 and IPv6 Headers",
RFC 2474, December 1998.
[DN-BIT] Rosen, E., Peter, P., and P. Pillay-Esnault,
"Using a Link State Advertisement (LSA) Options
Bit to Prevent Looping in BGP/MPLS IP Virtual
Private Networks (VPNs)", RFC 4576, June 2006.
[INTFMIB] McCloghrie, K. and F. Kastenholz, "The Interfaces
Group MIB", RFC 2863, June 2000.
[IP6ADDR] Hinden, R. and S. Deering, "IP Version 6
Addressing Architecture", RFC 4291, February 2006.
[IPAUTH] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[IPESP] Kent, S., "IP Encapsulating Security Payload
(ESP)", RFC 4303, December 2005.
[IPV4] Postal, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[IPV6] Deering, S. and R. Hinden, "Internet Protocol,
Version 6 (IPv6) Specification", RFC 2460,
December 1998.
[NSSA] Murphy, P., "The OSPF Not-So-Stubby Area (NSSA)
Option", RFC 3101, January 2003.
Coltun, et al. Standards Track [Page 55]
RFC 5340 OSPF for IPv6 July 2008
[OSPF-IANA] Kompella, K. and B. Fenner, "IANA Considerations
for OSPF", BCP 130, RFC 4940, July 2007.
[OSPFV2] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
April 1998.
[OSPFV3-AUTH] Gupta, M. and N. Melam, "Authentication/
Confidentiality for OSPFv3", RFC 4552, June 2006.
[RFC-KEYWORDS] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC 2119,
March 1997.
9.2. Informative References
[GENERIC-THREATS] Barbir, A., Murphy, S., and Y. Yang, "Generic
Threats to Routing Protocols", RFC 4593,
October 2006.
[MOSPF] Moy, J., "Multicast Extensions to OSPF", RFC 1584,
March 1994.
[MTUDISC] Mogul, J. and S. Deering, "Path MTU discovery",
RFC 1191, November 1990.
[OPAQUE] Coltun, R., "The OSPF Opaque LSA Option",
RFC 2370, July 1998.
[OSPFV3] Coltun, R., Ferguson, D., and J. Moy, "OSPF for
IPv6", RFC 2740, December 1999.
[OSPFV3-MIB] Joyal, D. and V. Manral, "Management Information
Base for OSPFv3", Work in Progress,
September 2007.
[SERV-CLASS] Babiarz, J., Chan, K., and F. Baker,
"Configuration Guidelines for DiffServ Service
Classes", RFC 4594, August 2006.
Coltun, et al. Standards Track [Page 56]
RFC 5340 OSPF for IPv6 July 2008
Appendix A. OSPF Data Formats
This appendix describes the format of OSPF protocol packets and OSPF
LSAs. The OSPF protocol runs directly over the IPv6 network layer.
Before any data formats are described, the details of the OSPF
encapsulation are explained.
Next, the OSPF Options field is described. This field describes
various capabilities that may or may not be supported by pieces of
the OSPF routing domain. The OSPF Options field is contained in OSPF
Hello packets, Database Description packets, and OSPF LSAs.
OSPF packet formats are detailed in Section A.3.
A description of OSPF LSAs appears in Section A.4. This section
describes how IPv6 address prefixes are represented within LSAs,
details the standard LSA header, and then provides formats for each
of the specific LSA types.
A.1. Encapsulation of OSPF Packets
OSPF runs directly over the IPv6's network layer. OSPF packets are
therefore encapsulated solely by IPv6 and local data-link headers.
OSPF does not define a way to fragment its protocol packets, and
depends on IPv6 fragmentation when transmitting packets larger than
the link MTU. If necessary, the length of OSPF packets can be up to
65,535 bytes. The OSPF packet types that are likely to be large
(Database Description, Link State Request, Link State Update, and
Link State Acknowledgment packets) can usually be split into multiple
protocol packets without loss of functionality. This is recommended;
IPv6 fragmentation should be avoided whenever possible. Using this
reasoning, an attempt should be made to limit the size of OSPF
packets sent over virtual links to 1280 bytes unless Path MTU
Discovery is being performed [MTUDISC].
The other important features of OSPF's IPv6 encapsulation are:
o Use of IPv6 multicast. Some OSPF messages are multicast when sent
over broadcast networks. Two distinct IP multicast addresses are
used. Packets sent to these multicast addresses should never be
forwarded; they are meant to travel a single hop only. As such,
the multicast addresses have been chosen with link-local scope and
packets sent to these addresses should have their IPv6 Hop Limit
set to 1. b
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RFC 5340 OSPF for IPv6 July 2008
AllSPFRouters
This multicast address has been assigned the value FF02::5.
All routers running OSPF should be prepared to receive packets
sent to this address. Hello packets are always sent to this
destination. Also, certain OSPF protocol packets are sent to
this address during the flooding procedure.
AllDRouters
This multicast address has been assigned the value FF02::6.
Both the Designated Router and Backup Designated Router must be
prepared to receive packets destined to this address. Certain
OSPF protocol packets are sent to this address during the
flooding procedure.
o OSPF is IP protocol 89. This number SHOULD be inserted in the
Next Header field of the encapsulating IPv6 header.
o The OSPFv2 specification (Appendix A.1 in [OSPFV2]) indicates that
OSPF protocol packets are sent with IP precedence set to
Internetwork Control (B'110') [IPV4]. If routers in the OSPF
routing domain map their IPv6 Traffic Class octet to the
Differentiated Services Code Point (DSCP) as specified in
[DIFF-SERV], then OSPFv3 packets SHOULD be sent with their DSCP
set to CS6 (B'110000'), as specified in [SERV-CLASS]. In networks
supporting this mapping, OSPF packets will be given precedence
over IPv6 data traffic.
A.2. The Options Field
The 24-bit OSPF Options field is present in OSPF Hello packets,
Database Description packets, and certain LSAs (router-LSAs, network-
LSAs, inter-area-router-LSAs, and link-LSAs). The Options field
enables OSPF routers to support (or not support) optional
capabilities, and to communicate their capability level to other OSPF
routers. Through this mechanism, routers of differing capabilities
can be mixed within an OSPF routing domain.
An option mismatch between routers can cause a variety of behaviors,
depending on the particular option. Some option mismatches prevent
neighbor relationships from forming (e.g., the E-bit below); these
mismatches are discovered through the sending and receiving of Hello
packets. Some option mismatches prevent particular LSA types from
being flooded across adjacencies; these are discovered through the
sending and receiving of Database Description packets. Some option
mismatches prevent routers from being included in one or more of the
various routing calculations because of their reduced functionality;
these mismatches are discovered by examining LSAs.
Coltun, et al. Standards Track [Page 58]
RFC 5340 OSPF for IPv6 July 2008
Seven bits of the OSPF Options field have been assigned. Each bit is
described briefly below. Routers should reset (i.e., clear)
unrecognized bits in the Options field when sending Hello packets or
Database Description packets and when originating LSAs. Conversely,
routers encountering unrecognized Options bits in received Hello
packets, Database Description packets, or LSAs should ignore the
unrecognized bits and process the packet or LSA normally.
1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+-+--+--+
| | | | | | | | | | | | | | | | |*|*|DC|R|N|x| E|V6|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+-+--+--+
The Options field
The Options field
V6-bit
If this bit is clear, the router/link should be excluded from IPv6
routing calculations. See Section 4.8 for details.
E-bit
This bit describes the way AS-external-LSAs are flooded, as
described in Sections 3.6, 9.5, 10.8, and 12.1.2 of [OSPFV2].
x-Bit
This bit was previously used by MOSPF (see [MOSPF]), which has
been deprecated for OSPFv3. The bit should be set to 0 and
ignored when received. It may be reassigned in the future.
N-bit
This bit indicates whether or not the router is attached to an
NSSA as specified in [NSSA].
R-bit
This bit (the `Router' bit) indicates whether the originator is an
active router. If the router bit is clear, then routes that
transit the advertising node cannot be computed. Clearing the
router bit would be appropriate for a multi-homed host that wants
to participate in routing, but does not want to forward non-
locally addressed packets.
DC-bit
This bit describes the router's handling of demand circuits, as
specified in [DEMAND].
Coltun, et al. Standards Track [Page 59]
RFC 5340 OSPF for IPv6 July 2008
*-bit
These bits are reserved for migration of OSPFv2 protocol
extensions.
A.3. OSPF Packet Formats
There are five distinct OSPF packet types. All OSPF packet types
begin with a standard 16-byte header. This header is described
first. Each packet type is then described in a succeeding section.
In these sections, each packet's format is displayed and the packet's
component fields are defined.
All OSPF packet types (other than the OSPF Hello packets) deal with
lists of LSAs. For example, Link State Update packets implement the
flooding of LSAs throughout the OSPF routing domain. The format of
LSAs is described in Section A.4.
The receive processing of OSPF packets is detailed in Section 4.2.2.
The sending of OSPF packets is explained in Section 4.2.1.
A.3.1. The OSPF Packet Header
Every OSPF packet starts with a standard 16-byte header. Together
with the encapsulating IPv6 headers, the OSPF header contains all the
information necessary to determine whether the packet should be
accepted for further processing. This determination is described in
Section 4.2.2.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version # | Type | Packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Instance ID | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The OSPF Packet Header
Version #
The OSPF version number. This specification documents version 3
of the OSPF protocol.
Coltun, et al. Standards Track [Page 60]
RFC 5340 OSPF for IPv6 July 2008
Type
The OSPF packet types are as follows. See Appendix A.3.2 through
Appendix A.3.6 for details.
Type Description
---------------------------------
1 Hello
2 Database Description
3 Link State Request
4 Link State Update
5 Link State Acknowledgment
Packet length
The length of the OSPF protocol packet in bytes. This length
includes the standard OSPF header.
Router ID
The Router ID of the packet's source.
Area ID
A 32-bit number identifying the area to which this packet belongs.
All OSPF packets are associated with a single area. Most travel a
single hop only. Packets traversing a virtual link are labeled
with the backbone Area ID of 0.
Checksum
OSPF uses the standard checksum calculation for IPv6 applications:
The 16-bit one's complement of the one's complement sum of the
entire contents of the packet, starting with the OSPF packet
header, and prepending a "pseudo-header" of IPv6 header fields, as
specified in Section 8.1 of [IPV6]. The "Upper-Layer Packet
Length" in the pseudo-header is set to the value of the OSPF
packet header's length field. The Next Header value used in the
pseudo-header is 89. If the packet's length is not an integral
number of 16-bit words, the packet is padded with a byte of zero
before checksumming. Before computing the checksum, the checksum
field in the OSPF packet header is set to 0.
Instance ID
Enables multiple instances of OSPF to be run over a single link.
Each protocol instance would be assigned a separate Instance ID;
the Instance ID has link-local significance only. Received
packets whose Instance ID is not equal to the receiving
interface's Instance ID are discarded.
Coltun, et al. Standards Track [Page 61]
RFC 5340 OSPF for IPv6 July 2008
0
These fields are reserved. They SHOULD be set to 0 when sending
protocol packets and MUST be ignored when receiving protocol
packets.
A.3.2. The Hello Packet
Hello packets are OSPF packet type 1. These packets are sent
periodically on all interfaces (including virtual links) in order to
establish and maintain neighbor relationships. In addition, Hello
packets are multicast on those links having a multicast or broadcast
capability, enabling dynamic discovery of neighboring routers.
All routers connected to a common link must agree on certain
parameters (HelloInterval and RouterDeadInterval). These parameters
are included in Hello packets allowing differences to inhibit the
forming of neighbor relationships. The Hello packet also contains
fields used in Designated Router election (Designated Router ID and
Backup Designated Router ID), and fields used to detect bidirectional
communication (the Router IDs of all neighbors whose Hellos have been
recently received).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 | 1 | Packet Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Instance ID | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rtr Priority | Options |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HelloInterval | RouterDeadInterval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Designated Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Backup Designated Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
The OSPF Hello Packet
Coltun, et al. Standards Track [Page 62]
RFC 5340 OSPF for IPv6 July 2008
Interface ID
32-bit number uniquely identifying this interface among the
collection of this router's interfaces. For example, in some
implementations it may be possible to use the MIB-II IfIndex
([INTFMIB]).
Rtr Priority
This router's Router Priority. Used in (Backup) Designated Router
election. If set to 0, the router will be ineligible to become
(Backup) Designated Router.
Options
The optional capabilities supported by the router, as documented
in Section A.2.
HelloInterval
The number of seconds between this router's Hello packets.
RouterDeadInterval
The number of seconds before declaring a silent router down.
Designated Router ID
The sending router's view of the identity of the Designated Router
for this network. The Designated Router is identified by its
Router ID. It is set to 0.0.0.0 if there is no Designated Router.
Backup Designated Router ID
The sending router's view of the identity of the Backup Designated
Router for this network. The Backup Designated Router is
identified by its IP Router ID. It is set to 0.0.0.0 if there is
no Backup Designated Router.
Neighbor ID
The Router IDs of each router on the network with neighbor state
1-Way or greater.
A.3.3. The Database Description Packet
Database Description packets are OSPF packet type 2. These packets
are exchanged when an adjacency is being initialized. They describe
the contents of the link-state database. Multiple packets may be
used to describe the database. For this purpose, a poll-response
procedure is used. One of the routers is designated to be the master
and the other is the slave. The master sends Database Description
packets (polls) that are acknowledged by Database Description packets
sent by the slave (responses). The responses are linked to the polls
via the packets' DD sequence numbers.
Coltun, et al. Standards Track [Page 63]
RFC 5340 OSPF for IPv6 July 2008
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+
| 3 | 2 | Packet Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+
| Checksum | Instance ID | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+
| 0 | Options |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+
| Interface MTU | 0 |0|0|0|0|0|I|M|MS|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+
| DD sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+
| |
+- -+
| |
+- An LSA Header -+
| |
+- -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+
| ... |
The OSPF Database Description Packet
The format of the Database Description packet is very similar to both
the Link State Request packet and the Link State Acknowledgment
packet. The main part of all three is a list of items, each item
describing a piece of the link-state database. The sending of
Database Description packets is documented in Section 10.8 of
[OSPFV2]. The reception of Database Description packets is
documented in Section 10.6 of [OSPFV2].
Options
The optional capabilities supported by the router, as documented
in Section A.2.
Interface MTU
The size in bytes of the largest IPv6 datagram that can be sent
out the associated interface without fragmentation. The MTUs of
common Internet link types can be found in Table 7-1 of [MTUDISC].
Coltun, et al. Standards Track [Page 64]
RFC 5340 OSPF for IPv6 July 2008
Interface MTU should be set to 0 in Database Description packets
sent over virtual links.
I-bit
The Init bit. When set to 1, this packet is the first in the
sequence of Database Description packets.
M-bit
The More bit. When set to 1, it indicates that more Database
Description packets are to follow.
MS-bit
The Master/Slave bit. When set to 1, it indicates that the router
is the master during the Database Exchange process. Otherwise,
the router is the slave.
DD sequence number
Used to sequence the collection of Database Description packets.
The initial value (indicated by the Init bit being set) should be
unique. The DD sequence number then increments until the complete
database for both the master and slave routers have been
exchanged.
The rest of the packet consists of a (possibly partial) list of the
link-state database's pieces. Each LSA in the database is described
by its LSA header. The LSA header is documented in Appendix A.4.2.
It contains all the information required to uniquely identify both
the LSA and the LSA's current instance.
A.3.4. The Link State Request Packet
Link State Request packets are OSPF packet type 3. After exchanging
Database Description packets with a neighboring router, a router may
find that parts of its link-state database are out-of-date. The Link
State Request packet is used to request the pieces of the neighbor's
database that are more up-to-date. Multiple Link State Request
packets may need to be used.
A router that sends a Link State Request packet has in mind the
precise instance of the database pieces it is requesting. Each
instance is defined by its LS sequence number, LS checksum, and LS
age, although these fields are not specified in the Link State
Request packet itself. The router may receive even more recent LSA
instances in response.
The sending of Link State Request packets is documented in Section
10.9 of [OSPFV2]. The reception of Link State Request packets is
documented in Section 10.7 of [OSPFV2].
Coltun, et al. Standards Track [Page 65]
RFC 5340 OSPF for IPv6 July 2008
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 | 3 | Packet Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Instance ID | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | LS Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
The OSPF Link State Request Packet
Each LSA requested is specified by its LS type, Link State ID, and
Advertising Router. This uniquely identifies the LSA without
specifying its instance. Link State Request packets are understood
to be requests for the most recent instance of the specified LSAs.
A.3.5. The Link State Update Packet
Link State Update packets are OSPF packet type 4. These packets
implement the flooding of LSAs. Each Link State Update packet
carries a collection of LSAs one hop further from their origin.
Several LSAs may be included in a single packet.
Link State Update packets are multicast on those physical networks
that support multicast/broadcast. In order to make the flooding
procedure reliable, flooded LSAs are acknowledged in Link State
Acknowledgment packets. If retransmission of certain LSAs is
necessary, the retransmitted LSAs are always carried by unicast Link
State Update packets. For more information on the reliable flooding
of LSAs, consult Section 4.5.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 | 4 | Packet Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Instance ID | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| # LSAs |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- +-+
| LSAs |
+- +-+
| ... |
The OSPF Link State Update Packet
# LSAs
The number of LSAs included in this update.
The body of the Link State Update packet consists of a list of LSAs.
Each LSA begins with a common 20-byte header, described in
Appendix A.4.2. Detailed formats of the different types of LSAs are
described Appendix A.4.
A.3.6. The Link State Acknowledgment Packet
Link State Acknowledgment packets are OSPF packet type 5. To make
the flooding of LSAs reliable, flooded LSAs are explicitly or
implicitly acknowledged. Explicit acknowledgment is accomplished
through the sending and receiving of Link State Acknowledgment
packets. The sending of Link State Acknowledgment packets is
documented in Section 13.5 of [OSPFV2]. The reception of Link State
Acknowledgment packets is documented in Section 13.7 of [OSPFV2].
Multiple LSAs MAY be acknowledged in a single Link State
Acknowledgment packet. Depending on the state of the sending
interface and the sender of the corresponding Link State Update
packet, a Link State Acknowledgment packet is sent to the multicast
address AllSPFRouters, the multicast address AllDRouters, or to a
neighbor's unicast address (see Section 13.5 of [OSPFV2] for
details).
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The format of this packet is similar to that of the Data Description
packet. The body of both packets is simply a list of LSA headers.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 | 5 | Packet Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Instance ID | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| |
+- An LSA Header -+
| |
+- -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
The OSPF Link State Acknowledgment Packet
Each acknowledged LSA is described by its LSA header. The LSA header
is documented in Appendix A.4.2. It contains all the information
required to uniquely identify both the LSA and the LSA's current
instance.
A.4. LSA Formats
This document defines eight distinct types of LSAs. Each LSA begins
with a standard 20-byte LSA header. This header is explained in
Appendix A.4.2. Succeeding sections describe each LSA type
individually.
Each LSA describes a piece of the OSPF routing domain. Every router
originates a router-LSA. A network-LSA is advertised for each link
by its Designated Router. A router's link-local addresses are
advertised to its neighbors in link-LSAs. IPv6 prefixes are
advertised in intra-area-prefix-LSAs, inter-area-prefix-LSAs, AS-
external-LSAs, and NSSA-LSAs. Location of specific routers can be
advertised across area boundaries in inter-area-router-LSAs. All
LSAs are then flooded throughout the OSPF routing domain. The
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flooding algorithm is reliable, ensuring that all routers common to a
flooding scope have the same collection of LSAs associated with that
flooding scope. (See Section 4.5 for more information concerning the
flooding algorithm.) This collection of LSAs is called the link-
state database.
From the link-state database, each router constructs a shortest-path
tree with itself as root. This yields a routing table (see Section
11 of [OSPFV2]). For details on the routing table build process, see
Section 4.8.
A.4.1. IPv6 Prefix Representation
IPv6 addresses are bit strings of length 128. IPv6 routing
protocols, and OSPF for IPv6 in particular, advertise IPv6 address
prefixes. IPv6 address prefixes are bit strings whose length ranges
between 0 and 128 bits (inclusive).
Within OSPF, IPv6 address prefixes are always represented by a
combination of three fields: PrefixLength, PrefixOptions, and Address
Prefix. PrefixLength is the length in bits of the prefix.
PrefixOptions is an 8-bit field describing various capabilities
associated with the prefix (see Appendix A.4.1.1). Address Prefix is
an encoding of the prefix itself as an even multiple of 32-bit words,
padding with zero bits as necessary. This encoding consumes
((PrefixLength + 31) / 32) 32-bit words.
The default route is represented by a prefix of length 0.
Examples of IPv6 Prefix representation in OSPF can be found in
Appendix A.4.5, Appendix A.4.7, Appendix A.4.8, Appendix A.4.9, and
Appendix A.4.10.
A.4.1.1. Prefix Options
Each prefix is advertised along with an 8-bit field of capabilities.
These serve as input to the various routing calculations. For
example, they can indicate that prefixes are to be ignored in some
cases or are to be marked as not readvertisable in others.
0 1 2 3 4 5 6 7
+--+--+--+--+--+-+--+--+
| | | |DN| P|x|LA|NU|
+--+--+--+--+--+-+--+--+
The PrefixOptions Field
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NU-bit
The "no unicast" capability bit. If set, the prefix should be
excluded from IPv6 unicast calculations. If not set, it should be
included.
LA-bit
The "local address" capability bit. If set, the prefix is
actually an IPv6 interface address of the Advertising Router.
Advertisement of local interface addresses is described in
Section 4.4.3.9. An implementation MAY also set the LA-bit for
prefixes advertised with a host PrefixLength (128).
x-bit
This bit was previously defined as a "multicast" capability bit.
However, the use was never adequately specified and has been
deprecated for OSPFv3. The bit should be set to 0 and ignored
when received. It may be reassigned in the future.
P-bit
The "propagate" bit. Set on NSSA area prefixes that should be
readvertised by the translating NSSA area border [NSSA].
DN-bit
This bit controls an inter-area-prefix-LSAs or AS-external-LSAs
re-advertisement in a VPN environment as specified in [DN-BIT].
A.4.2. The LSA Header
All LSAs begin with a common 20-byte header. This header contains
enough information to uniquely identify the LSA (LS type, Link State
ID, and Advertising Router). Multiple instances of the LSA may exist
in the routing domain at the same time. It is then necessary to
determine which instance is more recent. This is accomplished by
examining the LS age, LS sequence number, and LS checksum fields that
are also contained in the LSA header.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Age | LS Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The LSA Header
LS Age
The time in seconds since the LSA was originated.
LS Type
The LS type field indicates the function performed by the LSA.
The high-order three bits of LS type encode generic properties of
the LSA, while the remainder (called LSA function code) indicate
the LSA's specific functionality. See Appendix A.4.2.1 for a
detailed description of LS type.
Link State ID
The originating router's identifier for the LSA. The combination
of the Link State ID, LS type, and Advertising Router uniquely
identify the LSA in the link-state database.
Advertising Router
The Router ID of the router that originated the LSA. For example,
in network-LSAs this field is equal to the Router ID of the
network's Designated Router.
LS sequence number
Successive instances of an LSA are given successive LS sequence
numbers. The sequence number can be used to detect old or
duplicate LSA instances. See Section 12.1.6 in [OSPFV2] for more
details.
LS checksum
The Fletcher checksum of the complete contents of the LSA,
including the LSA header but excluding the LS age field. See
Section 12.1.7 in [OSPFV2] for more details.
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length
The length in bytes of the LSA. This includes the 20-byte LSA
header.
A.4.2.1. LSA Type
The LS type field indicates the function performed by the LSA. The
high-order three bits of LS type encode generic properties of the
LSA, while the remainder (called LSA function code) indicate the
LSA's specific functionality. The format of the LS type is as
follows:
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|U |S2|S1| LSA Function Code |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
LSA Type
The U-bit indicates how the LSA should be handled by a router that
does not recognize the LSA's function code. Its values are:
U-bit LSA Handling
-------------------------------------------------------------
0 Treat the LSA as if it had link-local flooding scope
1 Store and flood the LSA as if the type is understood
U-Bit
The S1 and S2 bits indicate the flooding scope of the LSA. The
values are:
S2 S1 Flooding Scope
-------------------------------------------------------------
0 0 Link-Local Scoping - Flooded only on originating link
0 1 Area Scoping - Flooded only in originating area
1 0 AS Scoping - Flooded throughout AS
1 1 Reserved
Flooding Scope
The LSA function codes are defined as follows. The origination and
processing of these LSA function codes are defined elsewhere in this
document, except for the NSSA-LSA (see [NSSA]) and 0x2006, which was
previously used by MOSPF (see [MOSPF]). MOSPF has been deprecated
for OSPFv3. As shown below, each LSA function b code also implies a
specific setting for the U, S1, and S2 bits.
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LSA Function Code LS Type Description
----------------------------------------------------
1 0x2001 Router-LSA
2 0x2002 Network-LSA
3 0x2003 Inter-Area-Prefix-LSA
4 0x2004 Inter-Area-Router-LSA
5 0x4005 AS-External-LSA
6 0x2006 Deprecated (may be reassigned)
7 0x2007 NSSA-LSA
8 0x0008 Link-LSA
9 0x2009 Intra-Area-Prefix-LSA
LSA Function Code
A.4.3. Router-LSAs
Router-LSAs have LS type equal to 0x2001. Each router in an area
originates one or more router-LSAs. The complete collection of
router-LSAs originated by the router describe the state and cost of
the router's interfaces to the area. For details concerning the
construction of router-LSAs, see Section 4.4.3.2. Router-LSAs are
only flooded throughout a single area.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Age |0|0|1| 1 |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Sequence Number |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Checksum | Length |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |Nt|x|V|E|B| Options |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | 0 | Metric |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface ID |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor Interface ID |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor Router ID |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | 0 | Metric |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface ID |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor Interface ID |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor Router ID |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Router-LSA Format
A single router may originate one or more router-LSAs, distinguished
by their Link State IDs (which are chosen arbitrarily by the
originating router). The Options field and V, E, and B bits should
be the same in all router-LSAs from a single originator. However, in
the case of a mismatch, the values in the LSA with the lowest Link
State ID take precedence. When more than one router-LSA is received
from a single router, the links are processed as if concatenated into
a single LSA.
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Bit V
When set, the router is an endpoint of one or more fully adjacent
virtual links having the described area as transit area (V is for
virtual link endpoint).
Bit E
When set, the router is an AS boundary router (E is for external).
Bit B
When set, the router is an area border router (B is for border).
Bit x
This bit was previously used by MOSPF (see [MOSPF]) and has been
deprecated for OSPFv3. The bit should be set to 0 and ignored
when received. It may be reassigned in the future.
Bit Nt
When set, the router is an NSSA border router that is
unconditionally translating NSSA-LSAs into AS-external-LSAs (Nt
stands for NSSA translation). Note that such routers have their
NSSATranslatorRole area configuration parameter set to Always.
(See [NSSA].)
Options
The optional capabilities supported by the router, as documented
in Appendix A.2.
The following fields are used to describe each router interface. The
Type field indicates the kind of interface being described. It may
be an interface to a transit network, a point-to-point connection to
another router, or a virtual link. The values of all the other
fields describing a router interface depend on the interface's Type
field.
Type
The kind of interface being described. One of the following:
Type Description
---------------------------------------------------
1 Point-to-point connection to another router
2 Connection to a transit network
3 Reserved
4 Virtual link
Router Link Types
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Metric
The cost of using this router interface for outbound traffic.
Interface ID
The Interface ID assigned to the interface being described. See
Section 4.1.2 and Appendix C.3.
Neighbor Interface ID
The Interface ID the neighbor router has associated with the link,
as advertised in the neighbor's Hello packets. For transit (type
2) links, the link's Designated Router is the neighbor described.
For other link types, the sole adjacent neighbor is described.
Neighbor Router ID
The Router ID the of the neighbor router. For transit (type 2)
links, the link's Designated Router is the neighbor described.
For other link types, the sole adjacent neighbor is described.
For transit (Type 2) links, the combination of Neighbor Interface ID
and Neighbor Router ID allows the network-LSA for the attached link
to be found in the link-state database.
A.4.4. Network-LSAs
Network-LSAs have LS type equal to 0x2002. A network-LSA is
originated for each broadcast and NBMA link in the area that includes
two or more adjacent routers. The network-LSA is originated by the
link's Designated Router. The LSA describes all routers attached to
the link including the Designated Router itself. The LSA's Link
State ID field is set to the Interface ID that the Designated Router
has been advertising in Hello packets on the link.
The distance from the network to all attached routers is zero. This
is why the Metric fields need not be specified in the network-LSA.
For details concerning the construction of network-LSAs, see
Section 4.4.3.3.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Age |0|0|1| 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | Options |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attached Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Network-LSA Format
Attached Router
The Router IDs of each of the routers attached to the link.
Actually, only those routers that are fully adjacent to the
Designated Router are listed. The Designated Router includes
itself in this list. The number of routers included can be
deduced from the LSA header's length field.
A.4.5. Inter-Area-Prefix-LSAs
Inter-area-prefix-LSAs have LS type equal to 0x2003. These LSAs are
the IPv6 equivalent of OSPF for IPv4's type 3 summary-LSAs (see
Section 12.4.3 of [OSPFV2]). Originated by area border routers, they
describe routes to IPv6 address prefixes that belong to other areas.
A separate inter-area-prefix-LSA is originated for each IPv6 address
prefix. For details concerning the construction of inter-area-
prefix-LSAs, see Section 4.4.3.4.
For stub areas, inter-area-prefix-LSAs can also be used to describe a
(per-area) default route. Default summary routes are used in stub
areas instead of flooding a complete set of external routes. When
describing a default summary route, the inter-area-prefix-LSA's
PrefixLength is set to 0.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Age |0|0|1| 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PrefixLength | PrefixOptions | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Prefix |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Inter-Area-Prefix-LSA Format
Metric
The cost of this route. Expressed in the same units as the
interface costs in router-LSAs. When the inter-area-prefix-LSA is
describing a route to a range of addresses (see Appendix C.2), the
cost is set to the maximum cost to any reachable component of the
address range.
PrefixLength, PrefixOptions, and Address Prefix
Representation of the IPv6 address prefix, as described in
Appendix A.4.1.
A.4.6. Inter-Area-Router-LSAs
Inter-area-router-LSAs have LS type equal to 0x2004. These LSAs are
the IPv6 equivalent of OSPF for IPv4's type 4 summary-LSAs (see
Section 12.4.3 of [OSPFV2]). Originated by area border routers, they
describe routes to AS boundary routers in other areas. To see why it
is necessary to advertise the location of each ASBR, consult Section
16.4 in [OSPFV2]. Each LSA describes a route to a single router.
For details concerning the construction of inter-area-router-LSAs,
see Section 4.4.3.5.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Age |0|0|1| 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | Options |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Inter-Area-Router-LSA Format
Options
The optional capabilities supported by the router, as documented
in Appendix A.2.
Metric
The cost of this route. Expressed in the same units as the
interface costs in router-LSAs.
Destination Router ID
The Router ID of the router being described by the LSA.
A.4.7. AS-External-LSAs
AS-external-LSAs have LS type equal to 0x4005. These LSAs are
originated by AS boundary routers and describe destinations external
to the AS. Each LSA describes a route to a single IPv6 address
prefix. For details concerning the construction of AS-external-LSAs,
see Section 4.4.3.6.
AS-external-LSAs can be used to describe a default route. Default
routes are used when no specific route exists to the destination.
When describing a default route, the AS-external-LSA's PrefixLength
is set to 0.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Age |0|1|0| 5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |E|F|T| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PrefixLength | PrefixOptions | Referenced LS Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Prefix |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| |
+- Forwarding Address (Optional) -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Route Tag (Optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Referenced Link State ID (Optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
AS-external-LSA Format
bit E
The type of external metric. If bit E is set, the metric
specified is a Type 2 external metric. This means the metric is
considered larger than any intra-AS path. If bit E is zero, the
specified metric is a Type 1 external metric. This means that it
is expressed in the same units as other LSAs (i.e., the same units
as the interface costs in router-LSAs).
bit F
If set, a Forwarding Address has been included in the LSA.
bit T
If set, an External Route Tag has been included in the LSA.
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Metric
The cost of this route. Interpretation depends on the external
type indication (bit E above).
PrefixLength, PrefixOptions, and Address Prefix
Representation of the IPv6 address prefix, as described in
Appendix A.4.1.
Referenced LS Type
If non-zero, an LSA with this LS type is to be associated with
this LSA (see Referenced Link State ID below).
Forwarding address
A fully qualified IPv6 address (128 bits). Included in the LSA if
and only if bit F has been set. If included, data traffic for the
advertised destination will be forwarded to this address. It MUST
NOT be set to the IPv6 Unspecified Address (0:0:0:0:0:0:0:0) or an
IPv6 Link-Local Address (Prefix FE80/10). While OSPFv3 routes are
normally installed with link-local addresses, an OSPFv3
implementation advertising a forwarding address MUST advertise a
global IPv6 address. This global IPv6 address may be the next-hop
gateway for an external prefix or may be obtained through some
other method (e.g., configuration).
External Route Tag
A 32-bit field that MAY be used to communicate additional
information between AS boundary routers. Included in the LSA if
and only if bit T has been set.
Referenced Link State ID
Included if and only if Reference LS Type is non-zero. If
included, additional information concerning the advertised
external route can be found in the LSA having LS type equal to
"Referenced LS Type", Link State ID equal to "Referenced Link
State ID", and Advertising Router the same as that specified in
the AS-external-LSA's link-state header. This additional
information is not used by the OSPF protocol itself. It may be
used to communicate information between AS boundary routers. The
precise nature of such information is outside the scope of this
specification.
All, none, or some of the fields labeled Forwarding address, External
Route Tag, and Referenced Link State ID MAY be present in the AS-
external-LSA (as indicated by the setting of bit F, bit T, and
Referenced LS Type respectively). When present, Forwarding Address
always comes first, External Route Tag next, and the Referenced Link
State ID last.
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A.4.8. NSSA-LSAs
NSSA-LSAs have LS type equal to 0x2007. These LSAs are originated by
AS boundary routers within an NSSA and describe destinations external
to the AS that may or may not be propagated outside the NSSA (refer
to [NSSA]). Other than the LS type, their format is exactly the same
as AS-external LSAs as described in Appendix A.4.7.
A global IPv6 address MUST be selected as forwarding address for
NSSA-LSAs that are to be propagated by NSSA area border routers. The
selection should proceed the same as OSPFv2 NSSA support [NSSA] with
additional checking to ensure IPv6 link-local address are not
selected.
A.4.9. Link-LSAs
Link-LSAs have LS type equal to 0x0008. A router originates a
separate link-LSA for each attached physical link. These LSAs have
link-local flooding scope; they are never flooded beyond the
associated link. Link-LSAs have three purposes:
1. They provide the router's link-local address to all other routers
attached to the link.
2. They inform other routers attached to the link of a list of IPv6
prefixes to associate with the link.
3. They allow the router to advertise a collection of Options bits
in the network-LSA originated by the Designated Router on a
broadcast or NBMA link.
For details concerning the construction of links-LSAs, see
Section 4.4.3.8.
A link-LSA's Link State ID is set equal to the originating router's
Interface ID on the link.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Age |0|0|0| 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rtr Priority | Options |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| |
+- Link-local Interface Address -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| # prefixes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PrefixLength | PrefixOptions | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Prefix |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PrefixLength | PrefixOptions | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Prefix |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Link-LSA Format
Rtr Priority
The Router Priority of the interface attaching the originating
router to the link.
Options
The set of Options bits that the router would like set in the
network-LSA that will be originated by the Designated Router on
broadcast or NBMA links.
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Link-local Interface Address
The originating router's link-local interface address on the link.
# prefixes
The number of IPv6 address prefixes contained in the LSA.
The rest of the link-LSA contains a list of IPv6 prefixes to be
associated with the link.
PrefixLength, PrefixOptions, and Address Prefix
Representation of an IPv6 address prefix, as described in
Appendix A.4.1.
A.4.10. Intra-Area-Prefix-LSAs
Intra-area-prefix-LSAs have LS type equal to 0x2009. A router uses
intra-area-prefix-LSAs to advertise one or more IPv6 address prefixes
that are associated with a local router address, an attached stub
network segment, or an attached transit network segment. In IPv4,
the first two were accomplished via the router's router-LSA and the
last via a network-LSA. In OSPF for IPv6, all addressing information
that was advertised in router-LSAs and network-LSAs has been removed
and is now advertised in intra-area-prefix-LSAs. For details
concerning the construction of intra-area-prefix-LSA, see
Section 4.4.3.9.
A router can originate multiple intra-area-prefix-LSAs for each
router or transit network. Each such LSA is distinguished by its
unique Link State ID.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Age |0|0|1| 9 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| # Prefixes | Referenced LS Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Referenced Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Referenced Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PrefixLength | PrefixOptions | Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Prefix |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PrefixLength | PrefixOptions | Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Prefix |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Intra-Area-Prefix LSA Format
# prefixes
The number of IPv6 address prefixes contained in the LSA.
Referenced LS Type, Referenced Link State ID, and Referenced
Advertising Router
Identifies the router-LSA or network-LSA with which the IPv6
address prefixes should be associated. If Referenced LS Type is
0x2001, the prefixes are associated with a router-LSA, Referenced
Link State ID should be 0, and Referenced Advertising Router
should be the originating router's Router ID. If Referenced LS
Type is 0x2002, the prefixes are associated with a network-LSA,
Referenced Link State ID should be the Interface ID of the link's
Designated Router, and Referenced Advertising Router should be the
Designated Router's Router ID.
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The rest of the intra-area-prefix-LSA contains a list of IPv6
prefixes to be associated with the router or transit link, as well as
their associated costs.
PrefixLength, PrefixOptions, and Address Prefix
Representation of an IPv6 address prefix, as described in
Appendix A.4.1.
Metric
The cost of this prefix. Expressed in the same units as the
interface costs in router-LSAs.
Appendix B. Architectural Constants
Architectural constants for the OSPF protocol are defined in Appendix
B of [OSPFV2]. The only difference for OSPF for IPv6 is that
DefaultDestination is encoded as a prefix with length 0 (see
Appendix A.4.1).
Appendix C. Configurable Constants
The OSPF protocol has quite a few configurable parameters. These
parameters are listed below. They are grouped into general
functional categories (area parameters, interface parameters, etc.).
Sample values are given for some of the parameters.
Some parameter settings need to be consistent among groups of
routers. For example, all routers in an area must agree on that
area's parameters. Similarly, all routers attached to a network must
agree on that network's HelloInterval and RouterDeadInterval.
Some parameters may be determined by router algorithms outside of
this specification (e.g., the address of a host connected to the
router via a SLIP line). From OSPF's point of view, these items are
still configurable.
C.1. Global Parameters
In general, a separate copy of the OSPF protocol is run for each
area. Because of this, most configuration parameters are defined on
a per-area basis. The few global configuration parameters are listed
below.
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Router ID
This is a 32-bit number that uniquely identifies the router in the
Autonomous System. If a router's OSPF Router ID is changed, the
router's OSPF software should be restarted before the new Router
ID takes effect. Before restarting due to a Router ID change, the
router should flush its self-originated LSAs from the routing
domain (see Section 14.1 of [OSPFV2]). Otherwise, they will
persist for up to MaxAge seconds.
Because the size of the Router ID is smaller than an IPv6 address, it
cannot be set to one of the router's IPv6 addresses (as is commonly
done for IPv4). Possible Router ID assignment procedures for IPv6
include: a) assign the IPv6 Router ID as one of the router's IPv4
addresses or b) assign IPv6 Router IDs through some local
administrative procedure (similar to procedures used by manufacturers
to assign product serial numbers).
The Router ID of 0.0.0.0 is reserved and SHOULD NOT be used.
C.2. Area Parameters
All routers belonging to an area must agree on that area's
configuration. Disagreements between two routers will lead to an
inability for adjacencies to form between them, with a resulting
hindrance to the flow of both routing protocol information and data
traffic. The following items must be configured for an area:
Area ID
This is a 32-bit number that identifies the area. The Area ID of
0 is reserved for the backbone.
List of address ranges
Address ranges control the advertisement of routes across area
boundaries. Each address range consists of the following items:
[IPv6 prefix, prefix length]
Describes the collection of IPv6 addresses contained in the
address range.
Status
Set to either Advertise or DoNotAdvertise. Routing information
is condensed at area boundaries. External to the area, at most
a single route is advertised (via a inter-area-prefix-LSA) for
each address range. The route is advertised if and only if the
address range's Status is set to Advertise. Unadvertised
ranges allow the existence of certain networks to be
intentionally hidden from other areas. Status is set to
Advertise by default.
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ExternalRoutingCapability
Whether AS-external-LSAs will be flooded into/throughout the area.
If AS-external-LSAs are excluded from the area, the area is called
a stub area or NSSA. Internal to stub areas, routing to external
destinations will be based solely on a default inter-area route.
The backbone cannot be configured as a stub or NSSA area. Also,
virtual links cannot be configured through stub or NSSA areas.
For more information, see Section 3.6 of [OSPFV2] and [NSSA].
StubDefaultCost
If the area has been configured as a stub area, and the router
itself is an area border router, then the StubDefaultCost
indicates the cost of the default inter-area-prefix-LSA that the
router should advertise into the area. See Section 12.4.3.1 of
[OSPFV2] for more information.
NSSATranslatorRole and TranslatorStabilityInterval
These area parameters are described in Appendix D of [NSSA].
Additionally, an NSSA Area Border Router (ABR) is also required to
allow configuration of whether or not an NSSA default route is
advertised in an NSSA-LSA. If advertised, its metric and metric
type are configurable. These requirements are also described in
Appendix D of [NSSA].
ImportSummaries
When set to enabled, prefixes external to the area are imported
into the area via the advertisement of inter-area-prefix-LSAs.
When set to disabled, inter-area routes are not imported into the
area. The default setting is enabled. This parameter is only
valid for stub or NSSA areas.
C.3. Router Interface Parameters
Some of the configurable router interface parameters (such as Area
ID, HelloInterval, and RouterDeadInterval) actually imply properties
of the attached links. Therefore, these parameters must be
consistent across all the routers attached to that link. The
parameters that must be configured for a router interface are:
IPv6 link-local address
The IPv6 link-local address associated with this interface. May
be learned through auto-configuration.
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Area ID
The OSPF area to which the attached link belongs.
Instance ID
The OSPF protocol instance associated with this OSPF interface.
Defaults to 0.
Interface ID
32-bit number uniquely identifying this interface among the
collection of this router's interfaces. For example, in some
implementations it may be possible to use the MIB-II IfIndex
([INTFMIB]).
IPv6 prefixes
The list of IPv6 prefixes to associate with the link. These will
be advertised in intra-area-prefix-LSAs.
Interface output cost(s)
The cost of sending a packet on the interface, expressed in the
link-state metric. This is advertised as the link cost for this
interface in the router's router-LSA. The interface output cost
MUST always be greater than 0.
RxmtInterval
The number of seconds between LSA retransmissions for adjacencies
belonging to this interface. Also used when retransmitting
Database Description and Link State Request packets. This should
be well over the expected round-trip delay between any two routers
on the attached link. The setting of this value should be
conservative or needless retransmissions will result. Sample
value for a local area network: 5 seconds.
InfTransDelay
The estimated number of seconds it takes to transmit a Link State
Update packet over this interface. LSAs contained in the update
packet must have their age incremented by this amount before
transmission. This value should take into account the
transmission and propagation delays of the interface. It MUST be
greater than 0. Sample value for a local area network: 1 second.
Router Priority
An 8-bit unsigned integer. When two routers attached to a network
both attempt to become the Designated Router, the one with the
highest Router Priority takes precedence. If there is still a
tie, the router with the highest Router ID takes precedence. A
router whose Router Priority is set to 0 is ineligible to become
the Designated Router on the attached link. Router Priority is
only configured for interfaces to broadcast and NBMA networks.
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HelloInterval
The length of time, in seconds, between Hello packets that the
router sends on the interface. This value is advertised in the
router's Hello packets. It MUST be the same for all routers
attached to a common link. The smaller the HelloInterval, the
faster topological changes will be detected. However, more OSPF
routing protocol traffic will ensue. Sample value for a X.25 PDN:
30 seconds. Sample value for a local area network (LAN): 10
seconds.
RouterDeadInterval
After ceasing to hear a router's Hello packets, the number of
seconds before its neighbors declare the router down. This is
also advertised in the router's Hello packets in their
RouterDeadInterval field. This should be some multiple of the
HelloInterval (e.g., 4). This value again MUST be the same for
all routers attached to a common link.
LinkLSASuppression
Indicates whether or not origination of a link-LSA is suppressed.
If set to "enabled" and the interface type is not broadcast or
NBMA, the router will not originate a link-LSA for the link. This
implies that other routers on the link will ascertain the router's
next-hop address using a mechanism other than the link-LSA (see
Section 4.8.2). The default value is "disabled" for interface
types described in this specification. It is implicitly
"disabled" if the interface type is broadcast or NBMA. Future
interface types MAY specify a different default.
C.4. Virtual Link Parameters
Virtual links are used to restore/increase connectivity of the
backbone. Virtual links may be configured between any pair of area
border routers having interfaces to a common (non-backbone) area.
The virtual link appears as a point-to-point link with no global IPv6
addresses in the graph for the backbone. The virtual link must be
configured in both of the area border routers.
A virtual link appears in router-LSAs (for the backbone) as if it
were a separate router interface to the backbone. As such, it has
most of the parameters associated with a router interface (see
Appendix C.3). Virtual links do not have link-local addresses, but
instead use one of the router's global-scope IPv6 addresses as the IP
source in OSPF protocol packets it sends on the virtual link. Router
Priority is not used on virtual links. Interface output cost is not
configured on virtual links, but is dynamically set to be the cost of
the transit area intra-area path between the two endpoint routers.
The parameter RxmtInterval may be configured and should be well over
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RFC 5340 OSPF for IPv6 July 2008
the expected round-trip delay between the two routers. This may be
hard to estimate for a virtual link; it is better to err on the side
of making it too long.
A virtual link is defined by the following two configurable
parameters: the Router ID of the virtual link's other endpoint and
the (non-backbone) area that the virtual link traverses (referred to
as the virtual link's transit area). Virtual links cannot be
configured through stub or NSSA areas. Additionally, an Instance ID
may be configured for virtual links from different protocol instances
in order to utilize the same transit area (without requiring
different Router IDs for demultiplexing).
C.5. NBMA Network Parameters
OSPF treats an NBMA network much like it treats a broadcast network.
Since there may be many routers attached to the network, a Designated
Router is selected for the network. This Designated Router then
originates a network-LSA listing all routers attached to the NBMA
network.
However, due to the lack of broadcast capabilities, it may be
necessary to use configuration parameters in the Designated Router
selection. These parameters will only need to be configured in those
routers that are themselves eligible to become the Designated Router
(i.e., those routers whose Router Priority for the network is non-
zero), and then only if no automatic procedure for discovering
neighbors exists:
List of all other attached routers
The list of all other routers attached to the NBMA network. Each
router is configured with its Router ID and IPv6 link-local
address on the network. Also, for each router listed, that
router's eligibility to become the Designated Router must be
defined. When an interface to an NBMA network first comes up, the
router only sends Hello packets to those neighbors eligible to
become the Designated Router until such time that a Designated
Router is elected.
PollInterval
If a neighboring router has become inactive (Hello packets have
not been seen for RouterDeadInterval seconds), it may still be
necessary to send Hello packets to the dead neighbor. These Hello
packets will be sent at the reduced rate PollInterval, which
should be much larger than HelloInterval. Sample value for a PDN
X.25 network: 2 minutes.
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C.6. Point-to-Multipoint Network Parameters
On point-to-multipoint networks, it may be necessary to configure the
set of neighbors that are directly reachable over the point-to-
multipoint network. Each neighbor is configured with its Router ID
and IPv6 link-local address on the network. Designated Routers are
not elected on point-to-multipoint networks, so the Designated Router
eligibility of configured neighbors is not defined.
C.7. Host Route Parameters
Host prefixes are advertised in intra-area-prefix-LSAs. They
indicate either local router addresses, router interfaces to point-
to-point networks, looped router interfaces, or IPv6 hosts that are
directly connected to the router (e.g., via a PPP connection). For
each host directly connected to the router, the following items must
be configured:
Host IPv6 prefix
An IPv6 prefix belonging to the directly connected host. This
must not be a valid IPv6 global prefix.
Cost of link to host
The cost of sending a packet to the host, in terms of the link-
state metric. However, since the host probably has only a single
connection to the Internet, the actual configured cost(s) in many
cases is unimportant (i.e., will have no effect on routing).
Area ID
The OSPF area to which the host's prefix belongs.
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Authors' Addresses
Rob Coltun
Acoustra Productions
3204 Brooklawn Terrace
Chevy Chase, MD 20815
USA
Dennis Ferguson
Juniper Networks
1194 N. Mathilda Avenue
Sunnyvale, CA 94089
USA
EMail: dennis@juniper.net
John Moy
Sycamore Networks, Inc
10 Elizabeth Drive
Chelmsford, MA 01824
USA
EMail: jmoy@sycamorenet.com
Acee Lindem (editor)
Redback Networks
102 Carric Bend Court
Cary, NC 27519
USA
EMail: acee@redback.com
Coltun, et al. Standards Track [Page 93]
RFC 5340 OSPF for IPv6 July 2008
Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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Coltun, et al. Standards Track [Page 94]