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Network Working Group R. Coltun
Request for Comments: 1587 RainbowBridge Communications
Category: Standards Track V. Fuller
Stanford University
March 1994
The OSPF NSSA Option
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.
Table Of Contents
1.0 Abstract ................................................. 1
2.0 Overview ................................................. 2
2.1 Motivation ............................................... 2
2.2 Proposed Solution ........................................ 3
3.0 Implementation Details ................................... 5
3.1 The N-bit ................................................ 5
3.2 Type-7 Address Ranges .................................... 5
3.3 Type-7 LSAs .............................................. 5
3.4 Originating Type-7 LSAs .................................. 7
3.5 Calculating Type-7 AS External Routes .................... 8
3.6 Incremental Updates ...................................... 10
4.0 Originating Type-5 LSAs .................................. 10
4.1 Translating Type-7 LSAs .................................. 10
4.2 Flushing Translated Type-7 LSAs .......................... 13
5.0 Acknowledgements ......................................... 13
6.0 References ............................................... 13
7.0 Security Considerations .................................. 13
8.0 Authors' Addresses ....................................... 14
Appendix A: Type-7 LSA Packet Format ......................... 15
Appendix B: The Options Field ................................ 16
Appendix C: Configuration Parameters ......................... 17
1.0 Abstract
This document describes a new optional type of OSPF area, somewhat
humorously referred to as a "not-so-stubby" area (or NSSA). NSSAs
are similar to the existing OSPF stub area configuration option but
have the additional capability of importing AS external routes in a
limited fashion.
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2.0 Overview
2.1 Motivation
Wide-area transit networks (such as the NSFNET regionals) often have
connections to moderately-complex "leaf" sites. A leaf site may have
multiple IP network numbers assigned to it.
Typically, one of the leaf site's networks is directly connected to a
router provided and administered by the transit network while the
others are distributed throughout and administered by the site. From
the transit network's perspective, all of the network numbers
associated with the site make up a single "stub" entity. For
example, BARRNet has one site composed of a class-B network,
130.57.0.0, and a class-C network, 192.31.114.0. From BARRNet's
perspective, this configuration looks something like this:
192.31.114
|
(cloud)
-------------- 130.57.4
|
|
------ 131.119.13 ------
|BR18|------------|BR10|
------ ------
|
V
to BARRNet "core" OSPF system
where the "cloud" consists of the subnets of 130.57 and network
192.31.114, all of which are learned by RIP on router BR18.
Topologically, this cloud looks very much like an OSPF stub area.
The advantages of running the cloud as an OSPF stub area are:
1. Type-5 routes (OSPF external link-state advertisements
(LSAs)) are not advertised beyond the router
labeled "BR10". This is advantageous because the
link between BR10 and BR18 may be a low-speed link
or the router BR18 may have limited resources.
2. The transit network is abstracted to the "leaf"
router BR18 by advertising only a default route
across the link between BR10 and BR18.
3. The cloud becomes a single, manageable "leaf" with
respect to the transit network.
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4. The cloud can become, logically, a part of the transit
network's OSPF routing system.
5. Translated type-5 LSAs that are sent into the
backbone from the cloud (which is a separate
stub area) may be considered "leaf" nodes
when performing the Dijkstra calculation.
However, the current definition of the OSPF protocol [1] imposes
topological limitations which restrict simple cloud topologies from
becoming OSPF stub areas. In particular, it is illegal for a stub
area to import routes external to OSPF; it is not possible for
routers BR18 and BR10 to both be members of the stub area and to
import the routes learned from RIP or other IP routing protocols as
type-5 (OSPF external LSAs) into the OSPF system. In order to run
OSPF out to BR18, BR18 must be a member of a non-stub area or the
OSPF backbone to import routes other than its directly-connected
network(s). Since it is not acceptable for BR18 to maintain all of
BARRNet's external (type-5) routes, BARRNet is forced by OSPF's
topological limitations to run OSPF out to BR10 and to run RIP
between BR18 and BR10.
2.2 Proposed Solution
This document describes a new optional type of OSPF area, somewhat
humorously referred to as a "not-so-stubby" area (or NSSA) which has
the capability of importing external routes in a limited fashion.
The OSPF specification defines two general classes of area
configuration. The first allows type-5 LSAs to be flooded throughout
the area. In this configuration, type-5 LSAs may be originated by
routers internal to the area or flooded into the area by area border
routers. These areas, referred to herein as type-5 capable areas (or
just plain areas in the OSPF spec), are distinguished by the fact
that they can carry transit traffic. The backbone is always a type-5
capable area. The second type of area configuration, called stub,
allows no type-5 LSAs to be propagated into/throughout the area and
instead depends on default routing to external destinations.
NSSAs are defined in much the same manner as existing stub areas. To
support NSSAs, a new option bit (the "N" bit) and a new type of LSA
(type-7) area defined. The "N" bit ensures that routers belonging to
an NSSA agree on its configuration. Similar to the stub area's use
of the "E" bit, both NSSA neighbors must agree on the setting of the
"N" bit or the OSPF neighbor adjacency will not form.
Type-7 LSAs provide for carrying external route information within an
NSSA. Type-7 AS External LSAs have virtually the same syntax as the
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Type-5 AS External LSAs with the obvious exception of the link-state
type (see section 3.2 for more details). There are two major semantic
differences between type-5 and type-7 LSAs.
o Type-7 LSAs may be originated by and advertised
throughout an NSSA; as with stub areas, NSSA's do not
receive or originate type-5 LSAs.
o Type-7 LSAs are advertised only within a single NSSA;
they are not flooded into the backbone area or any
other area by border routers, though the information
which they contain can be propagated into the backbone
area (see section 3.6).
In order to allow limited exchange of external information across an
NSSA area border, NSSA border routers will translate selected type-7
LSAs received from the NSSA into type-5 LSAs. These type-5 LSAs will
be flooded to all type-5 capable areas. NSSA area border routers may
be configured with address ranges so that several type-7 LSAs may be
represented by a single type-5 LSA.
In addition, an NSSA area border router can originate a default
type-7 LSA (IP address of 0.0.0.0) into the NSSA. Default routes are
necessary because NSSAs do not receive full routing information and
must have a default route to route to AS-external destinations. Like
stub areas, NSSAs may be connected to the backbone at more than one
area border router, but may not be used as a transit area. Note that
the default route originated by an NSSA area border router is never
translated into a type-5 LSA, however, a default route originated by
an NSSA internal AS boundary router (one that is not also an area
border router) may be translated into a type-5 LSA.
It should also be noted that unlike stub areas, all OSPF summary
routes (type-3 LSAs) must be imported into NSSAs. This is to ensure
that OSPF internal routes are always chosen over OSPF external
(type-7) routes.
In our example topology the subnets of 130.57 and network 192.31.114,
will still be learned by RIP on router BR18 but now both BR10 and
BR18 can be in an NSSA and all of BARRNets external routes are hidden
from BR18; BR10 becomes an NSSA area border router and BR18 becomes
an AS boundary router internal to the NSSA. BR18 will import the
subnets of 130.57 and network 192.31.114 as type-7 LSAs into the
NSSA. BR10 then translates these routes into type-5 LSAs and floods
them into BARRNet's backbone.
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3.0 Implementation Details
3.1 The N-bit
The N-bit ensures that all members of a NSSA agree on the area's
configuration. Together, the N-bit and E-bit reflect an interface's
(and consequently the interface's associated area's) external LSA
flooding capability. As explained in section 10.5 of the OSPF
specification, if type-5 LSAs are not flooded into/throughout the
area, the E-bit must be clear in the option field of the received
Hello packets. Interfaces associated with an NSSA will not send or
receive type-5 LSAs on that interface but may send and receive type-7
LSAs. Therefore, if the N-bit is set in the options field, the E-bit
must be cleared.
To support the NSSA option an additional check must be made in the
function that handles receiving Hello packet to verify that both the
N-bit and the E-bit found in the Hello packet's option field match
the value of the options that have been configured in the receiving
interface. A mismatch in the options causes processing of the
received Hello packet to stop and the packet to be dropped.
3.2 Type-7 Address Ranges
NSSA area border routers may be configured with type-7 address
ranges. Each address range is defined as an [address,mask] pair.
Many separate type-7 networks may then be represented by in a single
address range (as advertised by a type-5 LSA), just as a subnetted
network is composed of many separate subnets. Area border routers
may then summarize type-7 routes by advertising a single type-5 route
for each type-7 address range. The type-5 route, resulting from a
type-7 address range match will be distributed to all type-5 capable
areas. Section 4.1 gives the details of generating type-5 routes
from type-7 address ranges.
A type-7 address range includes the following configurable items.
o An [address,mask] pair.
o A status indication of either Advertise or
DoNotAdvertise.
o External route tag.
3.3 Type-7 LSAs: NSSA External Link-State Advertisements
External routes are imported into NSSAs as type-7 LSAs by the NSSA's
AS boundary routers. An NSSA AS boundary routers is a router which
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has an interface associated with the NSSA and is exchanging routing
information with routers belonging to another AS. As with type-5
LSAs a separate type-7 LSA is originated for each destination
network. To support NSSA areas, the link-state database must
therefore be expanded to contain a type-7 LSA.
Type 7-LSAs are identical to type-5 LSAs except for the following
(see section 12.3.4 "AS external links" in the OSPF
specification).
1. The type field in the LSA header is 7.
2. Type-7 LSAs are only flooded within the NSSA.
The flooding of type-7 LSAs follow the same rules
as the flooding of type 1-4 LSAs.
3. Type-7 LSAs are kept within the NSSA's LSDB (are
area specific) whereas because type-5 LSAs are
flooded to all type-5 capable areas, type-5 LSAs
global scope in the router's LSDB.
4. At the area border router, selected type-7 LSAs are
translated into type 5-LSAs and flooded into the
backbone.
5. Type 7 LSAs have a propagate (P) bit which is
used to flag the area border router to translate the
type-7 LSA into a type-5 LSA. Examples of how the P-bit
is used for loop avoidance are in the following sections.
6. Those type-7 LSAs that are to be translated into type-5
LSAs must have their forwarding address set.
Type-5 LSAs that have been translated from type-7 LSAs
for the most part must contain a forwarding address.
The execption to this is if the translation to a type-5
LSA is the result of an address range match, in which
case the type-5 LSA will not contain a forwarding address
(see section 4.1 for details).
The forwarding address contained in type-5 LSAs will
result in more efficient routing to the AS external
networks when there are multiple NSSA area
border routers. Having the forwarding address in the
type-7 LSAs will ease the translation of type-7 into
type-5 LSAs as the NSSA area border router will
not be required to compute the forwarding address.
If the network between the NSSA AS boundary router and the
adjacent AS is advertised into OSPF as an internal OSPF
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route, the forwarding address should be the next hop
address as is currently done in type-5 LSAs, but unlike
type-5 LSAs if the intervening network is not advertised
into OSPF as an internal OSPF route, the forwarding
address should be any one of the router's active OSPF
interface addresses.
Type-5 and type-7 metrics and path types are directly comparable.
3.4 Originating Type-7 LSAs
NSSA AS boundary routers may originate type-7 LSAs. All NSSA area
border routers must also be AS boundary routers since they all must
have the capability of translating a type-7 LSAs into a type-5 LSAs
(see section 3.6 routes for the translation algorithm). NSSA area
border routers must set the E-bit (external bit) as well as the B-bit
(border bit) in its router (type-1) LSAs (both in the backbone and in
the NSSA area).
When an NSSA internal AS boundary router originates a type-7 LSA that
it wants to be translated into a type-5 LSA by the NSSA area border
router (and subsequently flooded into the backbone), it must set the
P-bit in the LS header's option field and add a valid forwarding
address in the type-7 LSA.
If a router is attached to another AS and is also an NSSA area border
router, it may originate a both a type-5 and a type-7 LSA for the
same network. The type-5 LSA will be flooded to the backbone (and
all attached type-5 capable areas) and the type-7 will be flooded
into the NSSA. If this is the case, the P-bit must be reset in the
type-7 NSSA so the type-7 LSA isn't again translated into a type-5
LSA by another NSSA area border router.
A type-7 default route (network 0.0.0.0) may be originated into the
NSSA by an NSSA area border router or by an NSSA AS boundary router
which is internal to the NSSA. The type-7 default route originated
by the NSSA area border router must have the P-bit reset so that the
default route originated by the NSSA area border router will not find
its way out of the NSSA into the rest of the AS system via another
NSSA area border router. The type-7 default route originated by an
NSSA AS boundary router which is not an NSSA area border router may
have the P-bit set. Type-7 routes which are originated by the NSSA
area border router will not get added to other NSSA area border
router's routing table.
A default route must not be injected into the NSSA as a summary
(type-3) LSA as in the stub area case. The reason for this is that
the preferred summary default route would be chosen over all more
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specific type-7 routes. Because summary routes are preferred to
external routes and to ensure that summary routes are chosen over
external within the NSSA, all summary routes (unlike stub areas in
which this is optional) must be imported into an NSSA.
3.5 Calculating Type-7 AS External Routes
This section is very similar to section 16.4 (Calculating AS external
routes) in the OSPF specification. An NSSA area border router should
examine both type-5 LSAs and type-7 LSAs if either type-5 or type-7
routes need to be updated. Type-7 LSAs should be examined after
type-5 LSAs. An NSSA internal router should examine type-7 LSAs when
type-7 routes need to be recalculated.
In relation to the steps to calculate the routing table as presented
in the OSPF specification (chapter 16, "Calculation of the Routing
Table"), type-7 LSAs should be examined after step 5 where the routes
to external destinations are calculated.
Type-7 routes are calculated by examining type-7 LSAs. Each of LSAs
are considered in turn. Most type-7 LSAs describe routes to specific
IP destinations. A type-7 LSA can also describe a default route for
the NSSA (destination = DefaultDestination). For each type-7 LSA:
1. If the metric specified by the LSA is LSInfinity, the
age of the LSA equals MaxAge or the advertising router
field is equal to this router's router ID, examine the
next advertisement.
2. Call the destination described by the LSA N. Look up the
routing table entry for the AS boundary router (ASBR) that
originated the LSA. If no entry exists for the ASBR
(i.e., ASBR is unreachable), do nothing with this LSA and
consider the next in the list.
If the destination is the default route (destination =
DefaultDestination) and if the originator of the LSA and
the calculating router are both NSSA area border routers
do nothing with this LSA and consider the next in the list.
Else, this LSA describes an AS external path to destination
N. If the forwarding address (as specified in the forwarding
address field of the LSA) is 0.0.0.0, the packets routed
to the external destination N will be routed to the
originating ASBR. If the forwarding address is not 0.0.0.0,
look up the forwarding address in the routing table. Packets
routed to the external destination N will be routed within
the NSSA to this forwarding address. An intra-area path
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must therefore exist to the forwarding address. If no such
path exists, do nothing with the LSA and consider the next
in the list.
Call the routing table distance to the forwarding address
(or the distance to the originating ASBR if the forwarding
address is 0.0.0.0) X, and the cost specified in the type-7
LSA Y. X is in terms of the link-state metric, and Y is a
Type-1 or Type-2 external metric.
3. Now, look up the routing table entry for the destination
N. If no entries exist for N, install the AS external path
to N, with the next hop equal to the list of next hops to
the forwarding address/ASBR, and the advertising router equal
to ASBR. If the external metric type is 1, then the
path-type is set to Type-1 external and the cost is equal
to X + Y. If the external metric type is 2, the path-type
is set to Type-2 external, the link-state component of the
route's cost is X, and the Type-2 cost is Y.
4. Else, if the paths present in the table are not Type-1 or
Type-2 external paths, do nothing (AS external paths have
the lowest priority).
5. Otherwise, compare the cost of this new AS external path
to the ones present in the table. Note that type-5 and
type-7 routes are directly comparable. Type-1 external
paths are always shorter than Type-2 external paths.
Type-1 external paths are compared by looking at the sum
of the distance to the forwarding address/ASBR and the
advertised Type-1 paths (X+Y). Type-2 external paths are
compared by looking at the advertised Type-2 metrics,
and then if necessary, the distance to the forwarding
address/ASBR.
When a type-5 LSA and a type-7 LSA are found to have the
same type and an equal distance, the following priorities
apply (listed from highest to lowest) for breaking the tie.
a. Any type 5 LSA.
b. A type-7 LSA with the P-bit set and the forwarding
address non-zero.
c. Any other type-7 LSA.
If the new path is shorter, it replaces the present paths
in the routing table entry. If the new path is the same
cost, it is added to the routing table entry's list of
paths.
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3.6 Incremental Updates
Incremental updates for type-7 LSAs should be treated the same as
incremental updates for type-5 LSAs (see section 16.6 of the OSPF
specification). That is, if a new instance of a type-7 LSA is
received it is not necessary to recalculate the entire routing table.
If there is already an OSPF internal route to the destination
represented by the type-7 LSA, no recalculation is necessary.
Otherwise, the procedure in the proceeding section will have to be
performed but only for the external routes (type-5 and type-7) whose
networks describe the same networks as the newly received LSA.
4.0 Originating Type-5 LSAs
4.1 Translating Type-7 LSAs Into Type-5 LSAs
This step is performed as part of the NSSA's Dijkstra calculation
after type-5 and type-7 routes have been calculated. If the
calculating router is not an area border router this translation
algorithm should be skipped. All reachable area border routers in
the NSSA should now be examined noting the one with the highest
router ID. If this router has the highest router ID, it will be the
one translating type-7 LSAs into type-5 LSAs for the NSSA, otherwise
the translation algorithm should not be performed.
All type-7 routes that have been added to the routing table should be
examined. If the type-7 LSA (associated with the route being
examined) has the P-bit set and a non-zero forwarding address, the
following steps should be taken.
The translation procedure must first check for a configured type-7
address range. Recall that an type-7 address range consists of an
[address,mask] pair and a status indication of either Advertise or
DoNotAdvertise. At most a single type-5 LSA is made for each
range. If the route being examined falls within the type-7
address range, (the [address,mask] pair of the route equal to or a
more specific instance of the [address,mask] pair of the type-7
address range), one of following three actions may take place.
1. When the range's status indicates Advertise and the
route's address and mask are equal to the address
and mask of the type-7 range a type-5 LSA should be
originated if:
o there currently is no type-5 LSA originated from
this router corresponding to the type-7 LSA,
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o the path type or the metric in the corresponding
type-5 LSA is different from the type-7 LSA or
o The forwarding address in the corresponding
type-5 LSA is different from the type-7 LSA.
The newly originated type-5 LSA will describe
the same network and have the same network mask,
metrics, forwarding address, external route tag
and path type as the type-7 LSA, however, the
advertising router field will be the router ID
of this area border router.
2. When the range's status indicates Advertise and the
route's address or mask indicates a more specific
route (i.e., the route's address is subsumed by the
range or the route has a longer mask), a type-5 LSA
is generated with link-state ID equal to the range's
address (if necessary, the link-state ID can also have
one or more of the range's "host" bits set; see
Appendix F of the OSPF specification for details),
the network mask, external route tag and
path type will be set to the configured type-7 range
values. The advertising router field will be the
router ID of this area border router.
The forwarding address will not be set.
The path type should always be set to the highest
path type that is subsumed by the net range.
The metric for the type-5 LSA will be set as follows:
o if the path type is externl type 2, the type-5
metric should be set to the largest type-7 metric
subsumed by this net range + 1.
o if the path type is external type 1, the type-5
metric should be set to the largest metric.
For example, given a net range of [10.0.0.0, 255.0.0.0]
for an area that has type-7 routes of:
10.1.0.0 path type 1, metric 10
10.2.0.0 path type 1, metric 11
10.3.0.0 path type 2, metric 5
a type-5 LSA will be generated with a path type of 2
and a metric of 6.
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As another example, given a net range of
[10.0.0.0, 255.0.0.0] for an area that has
type-7 routes of:
10.1.0.0 path type 1, metric 10
10.2.0.0 path type 1, metric 11
10.3.0.0 path type 1, metric 5
a type-5 LSA will be generated with a path type of 1
and a metric of 11.
These metric and path type rules will avoid routing
loops in the event that path type 1 and 2 are both
used within the area.
3. When the range's status indicates DoNotAdvertise,
the type-5 LSA is suppressed and the component networks
remain hidden from the rest of the AS.
By default (given that the P-bit is set and the LSA has a
non-zero forwarding address) if a network is not contained
in any explicitly configured address range, a type-7 to
type-5 LSA translation will occur.
A new instance of a type-5 LSA should be originated and
flooded to all attached type-5 capable areas if one of the
following is true.
1. There currently is no type-5 LSA originated from this
router corresponding to the type-7 LSA.
2. The path type or the metric in the corresponding
type-5 LSA is different from the type-7 LSA.
3. The forwarding address in the corresponding
type-5 LSA is different from the type-7 LSA.
The newly originated type-5 LSAs will describe the same
network and have the same network mask, metrics, forwarding
address, external route tag and path type as the type-7 LSA.
The advertising router field will be the router ID of this
area border router.
As with all newly originated type-5 LSAs, a type-5 LSA that
is the result of a type-7 to type-5 translation (type-7 range
or default case) is flooded to all attached type-5 capable
areas.
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4.2 Flushing Translated Type-7 LSAs
If an NSSA area border router has translated a type-7 LSA to a type-5
LSA that should no longer be translated, the type-5 LSA should be
flushed (set to MaxAge and flooded). The translated type-5 LSA
should be flushed whenever the routing table entry that caused the
translation changes so that either the routing table entry is
unreachable or the entry's associated LSA is not a type-7 with the
P-bit set and a non-zero forwarding address.
5.0 Acknowledgements
This document was produced by the OSPF Working Group, chaired by John
Moy.
In addition, the comments of the following individuals are also
acknowledged:
Phani Jajjarvarpu cisco
Dino Farinacci cisco
Jeff Honig Cornell University
John Moy Proteon, Inc.
Doug Williams IBM
6.0 References
[1] Moy, J., "OSPF Version 2", RFC 1583, Proteon, Inc., March 1994.
[2] Moy, J., "Multicast Extensions to OSPF", RFC 1584, Proteon, Inc.,
Proteon, Inc., March 1994.
7.0 Security Considerations
Security issues are not discussed in this memo.
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8.0 Authors' Addresses
Rob Coltun
RainbowBridge Communications
Phone: (301) 340-9416
EMail: rcoltun@rainbow-bridge.com
Vince Fuller
BARRNet
Stanford University
Pine Hall 115
Stanford, CA, 94305-4122
Phone: (415) 723-6860
EMail: vaf@Valinor.Stanford.EDU
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Appendix A: Type-7 Packet Format
0 32
-----------------------------------
| | OPTS | 7 |
| ------------------
| Link-State Header |
| |
-----------------------------------
| Network Mask |
----------------------------------- ______
|E| Tos | metric | .
----------------------------------- . repeated for each TOS
| Forwarding Address | .
----------------------------------- .
| External Route Tag | ______
-----------------------------------
The definitions of the link-state ID, network mask, metrics and
external route tag are the same as the definitions for the type-5
LSAs (see A.4.5 in the OSPF specification) except for:
The Forwarding Address
If the network between the NSSA AS boundary router and the adjacent
AS is advertised into OSPF as an internal OSPF route, the forwarding
address should be the next hop address but if the intervening network
is not advertised into OSPF as an internal OSPF route, the forwarding
address should be any one of the router's active OSPF interface
addresses.
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Appendix B: The Options Field
The OSPF options field is present in OSPF Hello packets, Database
Description packets and all link-state advertisements. See appendix
A.2 in the OSPF specification for a description of option field.
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| * | * | * | * | N/P | MC | E | T |
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The Type-7 LSA options field
T-bit: The T-bit describes the router's TOS capability.
E-bit: Type-5 AS external link advertisements are not
flooded into/through OSPF stub and NSSA areas.
The E-bit ensures that all members of a stub area
agree on that area configuration. The E-bit is
meaningful only in OSPF Hello packets. When the
E-bit is reset in the Hello packet sent out a
particular interface, it means that the router
will neither send nor receive type-5 AS external
link state advertisements on that interface (in
other words, the interface connects to a stub
area). Two routers will not become neighbors
unless they agree on the state of the E-bit.
MC-bit: The MC-bit describes the multicast capability of
the various pieces of the OSPF routing domain
[2].
N-bit: The N-bit describes the the router's NSSA
capability. The N-bit is used only in Hello
packets and ensures that all members of an NSSA
agree on that area's configuration. When the
N-bit is reset in the Hello packet sent out a
particular interface, it means that the router
will neither send nor receive type-7 LSAs on that
interface. Two routers will not form an adjacency
unless they agree on the state of the N-bit. If
the N-bit is set in the options field, the E-bit
must be reset.
P-bit: The P-bit is used only in the type-7 LSA header.
It flags the NSSA area border router to translate
the type-7 LSA into a type-5 LSA.
Coltun & Fuller [Page 16]
RFC 1587 OSPF NSSA Option March 1994
Appendix C: Configuration Parameters
Appendix C.2 in the OSPF specification lists the area parameters.
The area ID, list of address ranges for type-3 summary routes and
authentication type remain unchanged. Section 3.2 of this document
lists the configuration parameters for type-7 address ranges.
For NSSAs the external capabilities of the area must be set to accept
type-7 external routes. Additionally there must be a way of
configuring the NSSA area border router to send a default route into
the NSSA using a specific metric (type-1 or type-2 and the actual
cost).
Coltun & Fuller [Page 17]