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Network Working Group R. Hinden, Ed.
Request for Comments: 3768 Nokia
Obsoletes: 2338 April 2004
Category: Standards Track
Virtual Router Redundancy Protocol (VRRP)
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.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This memo defines the Virtual Router Redundancy Protocol (VRRP).
VRRP specifies an election protocol that dynamically assigns
responsibility for a virtual router to one of the VRRP routers on a
LAN. The VRRP router controlling the IP address(es) associated with
a virtual router is called the Master, and forwards packets sent to
these IP addresses. The election process provides dynamic fail over
in the forwarding responsibility should the Master become
unavailable. This allows any of the virtual router IP addresses on
the LAN to be used as the default first hop router by end-hosts. The
advantage gained from using VRRP is a higher availability default
path without requiring configuration of dynamic routing or router
discovery protocols on every end-host.
Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Contributors. . . . . . . . . . . . . . . . . . . . . . 3
1.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Definitions . . . . . . . . . . . . . . . . . . . . . . 4
2. Required Features . . . . . . . . . . . . . . . . . . . . . . 5
2.1. IP Address Backup . . . . . . . . . . . . . . . . . . . 5
2.2. Preferred Path Indication . . . . . . . . . . . . . . . 5
2.3. Minimization of Unnecessary Service Disruptions . . . . 5
2.4. Efficient Operation over Extended LANs. . . . . . . . . 6
3. VRRP Overview . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Sample Configurations . . . . . . . . . . . . . . . . . . . . 7
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4.1. Sample Configuration 1. . . . . . . . . . . . . . . . . 7
4.2. Sample Configuration 2. . . . . . . . . . . . . . . . . 9
5. Protocol. . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. VRRP Packet Format. . . . . . . . . . . . . . . . . . . 10
5.2. IP Field Descriptions . . . . . . . . . . . . . . . . . 10
5.3. VRRP Field Descriptions . . . . . . . . . . . . . . . . 11
6. Protocol State Machine. . . . . . . . . . . . . . . . . . . . 13
6.1. Parameters per Virtual Router . . . . . . . . . . . . . 13
6.2. Timers. . . . . . . . . . . . . . . . . . . . . . . . . 14
6.3. State Transition Diagram. . . . . . . . . . . . . . . . 15
6.4. State Descriptions. . . . . . . . . . . . . . . . . . . 15
7. Sending and Receiving VRRP Packets. . . . . . . . . . . . . . 18
7.1. Receiving VRRP Packets. . . . . . . . . . . . . . . . . 18
7.2. Transmitting Packets. . . . . . . . . . . . . . . . . . 19
7.3. Virtual MAC Address . . . . . . . . . . . . . . . . . . 19
8. Operational Issues. . . . . . . . . . . . . . . . . . . . . . 20
8.1. ICMP Redirects. . . . . . . . . . . . . . . . . . . . . 20
8.2. Host ARP Requests . . . . . . . . . . . . . . . . . . . 20
8.3. Proxy ARP . . . . . . . . . . . . . . . . . . . . . . . 20
8.4. Potential Forwarding Loop . . . . . . . . . . . . . . . 21
9. Operation over FDDI, Token Ring, and ATM LANE . . . . . . . . 21
9.1. Operation over FDDI . . . . . . . . . . . . . . . . . . 21
9.2. Operation over Token Ring . . . . . . . . . . . . . . . 21
9.3. Operation over ATM LANE . . . . . . . . . . . . . . . . 23
10. Security Considerations . . . . . . . . . . . . . . . . . . . 23
11. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . 24
12. References. . . . . . . . . . . . . . . . . . . . . . . . . . 24
12.1. Normative References. . . . . . . . . . . . . . . . . . 24
12.2. Informative References. . . . . . . . . . . . . . . . . 25
13. Changes from RFC2338. . . . . . . . . . . . . . . . . . . . . 25
14. Editor's Address. . . . . . . . . . . . . . . . . . . . . . . 26
15. Full Copyright Statement. . . . . . . . . . . . . . . . . . . 27
1. Introduction
There are a number of methods that an end-host can use to determine
its first hop router towards a particular IP destination. These
include running (or snooping) a dynamic routing protocol such as
Routing Information Protocol [RIP] or OSPF version 2 [OSPF], running
an ICMP router discovery client [DISC] or using a statically
configured default route.
Running a dynamic routing protocol on every end-host may be
infeasible for a number of reasons, including administrative
overhead, processing overhead, security issues, or lack of a protocol
implementation for some platforms. Neighbor or router discovery
protocols may require active participation by all hosts on a network,
leading to large timer values to reduce protocol overhead in the face
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of large numbers of hosts. This can result in a significant delay in
the detection of a lost (i.e., dead) neighbor, that may introduce
unacceptably long "black hole" periods.
The use of a statically configured default route is quite popular; it
minimizes configuration and processing overhead on the end-host and
is supported by virtually every IP implementation. This mode of
operation is likely to persist as dynamic host configuration
protocols [DHCP] are deployed, which typically provide configuration
for an end-host IP address and default gateway. However, this
creates a single point of failure. Loss of the default router
results in a catastrophic event, isolating all end-hosts that are
unable to detect any alternate path that may be available.
The Virtual Router Redundancy Protocol (VRRP) is designed to
eliminate the single point of failure inherent in the static default
routed environment. VRRP specifies an election protocol that
dynamically assigns responsibility for a virtual router to one of the
VRRP routers on a LAN. The VRRP router controlling the IP
address(es) associated with a virtual router is called the Master,
and forwards packets sent to these IP addresses. The election
process provides dynamic fail-over in the forwarding responsibility
should the Master become unavailable. Any of the virtual router's IP
addresses on a LAN can then be used as the default first hop router
by end-hosts. The advantage gained from using VRRP is a higher
availability default path without requiring configuration of dynamic
routing or router discovery protocols on every end-host.
VRRP provides a function similar to the proprietary protocols "Hot
Standby Router Protocol (HSRP)" [HSRP] and "IP Standby Protocol"
[IPSTB].
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 [RFC2119].
1.1. Contributors
The following people, who are the authors of the RFC 2338 that this
document is based on and replaces, contributed to the text in this
document. They are P. Higginson, R. Hinden, P. Hunt, S. Knight, A.
Lindem, D. Mitzel, M. Shand, D. Weaver, and D. Whipple. They are not
listed as authors of the document due to current RFC-Editor policies.
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1.2. Scope
The remainder of this document describes the features, design goals,
and theory of operation of VRRP. The message formats, protocol
processing rules and state machine that guarantee convergence to a
single Virtual Router Master are presented. Finally, operational
issues related to MAC address mapping, handling of ARP requests,
generation of ICMP redirect messages, and security issues are
addressed.
This protocol is intended for use with IPv4 routers only. A separate
specification will be produced if it is decided that similar
functionality is desirable in an IPv6 environment.
1.3. Definitions
VRRP Router A router running the Virtual Router Redundancy
Protocol. It may participate in one or more
virtual routers.
Virtual Router An abstract object managed by VRRP that acts
as a default router for hosts on a shared LAN.
It consists of a Virtual Router Identifier and
a set of associated IP address(es) across a
common LAN. A VRRP Router may backup one or
more virtual routers.
IP Address Owner The VRRP router that has the virtual router's
IP address(es) as real interface address(es).
This is the router that, when up, will respond
to packets addressed to one of these IP
addresses for ICMP pings, TCP connections,
etc.
Primary IP Address An IP address selected from the set of real
interface addresses. One possible selection
algorithm is to always select the first
address. VRRP advertisements are always sent
using the primary IP address as the source of
the IP packet.
Virtual Router Master The VRRP router that is assuming the
responsibility of forwarding packets sent to
the IP address(es) associated with the virtual
router, and answering ARP requests for these
IP addresses. Note that if the IP address
owner is available, then it will always become
the Master.
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Virtual Router Backup The set of VRRP routers available to assume
forwarding responsibility for a virtual router
should the current Master fail.
2. Required Features
This section outlines the set of features that were considered
mandatory and that guided the design of VRRP.
2.1. IP Address Backup
Backup of IP addresses is the primary function of the Virtual Router
Redundancy Protocol. While providing election of a Virtual Router
Master and the additional functionality described below, the protocol
should strive to:
- Minimize the duration of black holes.
- Minimize the steady state bandwidth overhead and processing
complexity.
- Function over a wide variety of multiaccess LAN technologies
capable of supporting IP traffic.
- Provide for election of multiple virtual routers on a network for
load balancing.
- Support of multiple logical IP subnets on a single LAN segment.
2.2. Preferred Path Indication
A simple model of Master election among a set of redundant routers is
to treat each router with equal preference and claim victory after
converging to any router as Master. However, there are likely to be
many environments where there is a distinct preference (or range of
preferences) among the set of redundant routers. For example, this
preference may be based upon access link cost or speed, router
performance or reliability, or other policy considerations. The
protocol should allow the expression of this relative path preference
in an intuitive manner, and guarantee Master convergence to the most
preferential router currently available.
2.3. Minimization of Unnecessary Service Disruptions
Once Master election has been performed then any unnecessary
transitions between Master and Backup routers can result in a
disruption in service. The protocol should ensure after Master
election that no state transition is triggered by any Backup router
of equal or lower preference as long as the Master continues to
function properly.
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Some environments may find it beneficial to avoid the state
transition triggered when a router becomes available that is
preferred over the current Master. It may be useful to support an
override of the immediate convergence to the preferred path.
2.4. Efficient Operation over Extended LANs
Sending IP packets on a multiaccess LAN requires mapping from an IP
address to a MAC address. The use of the virtual router MAC address
in an extended LAN employing learning bridges can have a significant
effect on the bandwidth overhead of packets sent to the virtual
router. If the virtual router MAC address is never used as the
source address in a link level frame then the station location is
never learned, resulting in flooding of all packets sent to the
virtual router. To improve the efficiency in this environment the
protocol should: 1) use the virtual router MAC as the source in a
packet sent by the Master to trigger station learning; 2) trigger a
message immediately after transitioning to Master to update the
station learning; and 3) trigger periodic messages from the Master to
maintain the station learning cache.
3. VRRP Overview
VRRP specifies an election protocol to provide the virtual router
function described earlier. All protocol messaging is performed
using IP multicast datagrams, thus the protocol can operate over a
variety of multiaccess LAN technologies supporting IP multicast.
Each VRRP virtual router has a single well-known MAC address
allocated to it. This document currently only details the mapping to
networks using the IEEE 802 48-bit MAC address. The virtual router
MAC address is used as the source in all periodic VRRP messages sent
by the Master router to enable bridge learning in an extended LAN.
A virtual router is defined by its virtual router identifier (VRID)
and a set of IP addresses. A VRRP router may associate a virtual
router with its real addresses on an interface, and may also be
configured with additional virtual router mappings and priority for
virtual routers it is willing to backup. The mapping between VRID
and addresses must be coordinated among all VRRP routers on a LAN.
However, there is no restriction against reusing a VRID with a
different address mapping on different LANs. The scope of each
virtual router is restricted to a single LAN.
To minimize network traffic, only the Master for each virtual router
sends periodic VRRP Advertisement messages. A Backup router will not
attempt to preempt the Master unless it has higher priority. This
eliminates service disruption unless a more preferred path becomes
available. It's also possible to administratively prohibit all
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preemption attempts. The only exception is that a VRRP router will
always become Master of any virtual router associated with addresses
it owns. If the Master becomes unavailable then the highest priority
Backup will transition to Master after a short delay, providing a
controlled transition of the virtual router responsibility with
minimal service interruption.
The VRRP protocol design provides rapid transition from Backup to
Master to minimize service interruption, and incorporates
optimizations that reduce protocol complexity while guaranteeing
controlled Master transition for typical operational scenarios. The
optimizations result in an election protocol with minimal runtime
state requirements, minimal active protocol states, and a single
message type and sender. The typical operational scenarios are
defined to be two redundant routers and/or distinct path preferences
among each router. A side effect when these assumptions are violated
(i.e., more than two redundant paths all with equal preference) is
that duplicate packets may be forwarded for a brief period during
Master election. However, the typical scenario assumptions are
likely to cover the vast majority of deployments, loss of the Master
router is infrequent, and the expected duration in Master election
convergence is quite small ( << 1 second ). Thus the VRRP
optimizations represent significant simplifications in the protocol
design while incurring an insignificant probability of brief network
degradation.
4. Sample Configurations
4.1. Sample Configuration 1
The following figure shows a simple network with two VRRP routers
implementing one virtual router. Note that this example is provided
to help understand the protocol, but is not expected to occur in
actual practice.
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+-----------+ +-----------+
| Rtr1 | | Rtr2 |
|(MR VRID=1)| |(BR VRID=1)|
| | | |
VRID=1 +-----------+ +-----------+
IP A ---------->* *<--------- IP B
| |
| |
------------------+------------+-----+--------+--------+--------+--
^ ^ ^ ^
| | | |
(IP A) (IP A) (IP A) (IP A)
| | | |
+--+--+ +--+--+ +--+--+ +--+--+
| H1 | | H2 | | H3 | | H4 |
+-----+ +-----+ +--+--+ +--+--+
Legend:
---+---+---+-- = Ethernet, Token Ring, or FDDI
H = Host computer
MR = Master Router
BR = Backup Router
* = IP Address
(IP) = default router for hosts
Eliminating all mention of VRRP (VRID=1) from the figure above leaves
it as a typical IP deployment. Each router is permanently assigned
an IP address on the LAN interface (Rtr1 is assigned IP A and Rtr2 is
assigned IP B), and each host installs a static default route through
one of the routers (in this example they all use Rtr1's IP A).
Moving to the VRRP environment, each router has the exact same
permanently assigned IP address. Rtr1 is said to be the IP address
owner of IP A, and Rtr2 is the IP address owner of IP B. A virtual
router is then defined by associating a unique identifier (the
virtual router ID) with the address owned by a router. Finally, the
VRRP protocol manages virtual router fail over to a backup router.
The example above shows a virtual router configured to cover the IP
address owned by Rtr1 (VRID=1,IP_Address=A). When VRRP is enabled on
Rtr1 for VRID=1 it will assert itself as Master, with priority=255,
since it is the IP address owner for the virtual router IP address.
When VRRP is enabled on Rtr2 for VRID=1 it will transition to Backup,
with priority=100, since it is not the IP address owner. If Rtr1
should fail then the VRRP protocol will transition Rtr2 to Master,
temporarily taking over forwarding responsibility for IP A to provide
uninterrupted service to the hosts.
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Note that in this example IP B is not backed up, it is only used by
Rtr2 as its interface address. In order to backup IP B, a second
virtual router must be configured. This is shown in the next
section.
4.2. Sample Configuration 2
The following figure shows a configuration with two virtual routers
with the hosts spitting their traffic between them. This example is
expected to be very common in actual practice.
+-----------+ +-----------+
| Rtr1 | | Rtr2 |
|(MR VRID=1)| |(BR VRID=1)|
|(BR VRID=2)| |(MR VRID=2)|
VRID=1 +-----------+ +-----------+ VRID=2
IP A ---------->* *<---------- IP B
| |
| |
------------------+------------+-----+--------+--------+--------+--
^ ^ ^ ^
| | | |
(IP A) (IP A) (IP B) (IP B)
| | | |
+--+--+ +--+--+ +--+--+ +--+--+
| H1 | | H2 | | H3 | | H4 |
+-----+ +-----+ +--+--+ +--+--+
Legend:
---+---+---+-- = Ethernet, Token Ring, or FDDI
H = Host computer
MR = Master Router
BR = Backup Router
* = IP Address
(IP) = default router for hosts
In the example above, half of the hosts have configured a static
route through Rtr1's IP A and half are using Rtr2's IP B. The
configuration of virtual router VRID=1 is exactly the same as in the
first example (see section 4.1), and a second virtual router has been
added to cover the IP address owned by Rtr2 (VRID=2, IP_Address=B).
In this case Rtr2 will assert itself as Master for VRID=2 while Rtr1
will act as a backup. This scenario demonstrates a deployment
providing load splitting when both routers are available while
providing full redundancy for robustness.
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5. Protocol
The purpose of the VRRP packet is to communicate to all VRRP routers
the priority and the state of the Master router associated with the
Virtual Router ID.
VRRP packets are sent encapsulated in IP packets. They are sent to
the IPv4 multicast address assigned to VRRP.
5.1. VRRP Packet Format
This section defines the format of the VRRP packet and the relevant
fields in the IP header.
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 | Virtual Rtr ID| Priority | Count IP Addrs|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Auth Type | Adver Int | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP Address (1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP Address (n) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication Data (1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication Data (2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.2. IP Field Descriptions
5.2.1. Source Address
The primary IP address of the interface the packet is being sent
from.
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5.2.2. Destination Address
The IP multicast address as assigned by the IANA for VRRP is:
224.0.0.18
This is a link local scope multicast address. Routers MUST NOT
forward a datagram with this destination address regardless of its
TTL.
5.2.3. TTL
The TTL MUST be set to 255. A VRRP router receiving a packet with
the TTL not equal to 255 MUST discard the packet.
5.2.4. Protocol
The IP protocol number assigned by the IANA for VRRP is 112
(decimal).
5.3. VRRP Field Descriptions
5.3.1. Version
The version field specifies the VRRP protocol version of this packet.
This document defines version 2.
5.3.2. Type
The type field specifies the type of this VRRP packet. The only
packet type defined in this version of the protocol is:
1 ADVERTISEMENT
A packet with unknown type MUST be discarded.
5.3.3. Virtual Rtr ID (VRID)
The Virtual Router Identifier (VRID) field identifies the virtual
router this packet is reporting status for. Configurable item in the
range 1-255 (decimal). There is no default.
5.3.4. Priority
The priority field specifies the sending VRRP router's priority for
the virtual router. Higher values equal higher priority. This field
is an 8 bit unsigned integer field.
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The priority value for the VRRP router that owns the IP address(es)
associated with the virtual router MUST be 255 (decimal).
VRRP routers backing up a virtual router MUST use priority values
between 1-254 (decimal). The default priority value for VRRP routers
backing up a virtual router is 100 (decimal).
The priority value zero (0) has special meaning indicating that the
current Master has stopped participating in VRRP. This is used to
trigger Backup routers to quickly transition to Master without having
to wait for the current Master to timeout.
5.3.5. Count IP Addrs
The number of IP addresses contained in this VRRP advertisement.
5.3.6. Authentication Type
The authentication type field identifies the authentication method
being utilized. Authentication type is unique on a Virtual Router
basis. The authentication type field is an 8 bit unsigned integer.
A packet with unknown authentication type or that does not match the
locally configured authentication method MUST be discarded.
Note: Earlier version of the VRRP specification had several defined
authentication types [RFC2338]. These were removed in this
specification because operational experience showed that they did not
provide any real security and would only cause multiple masters to be
created.
The authentication methods currently defined are:
0 - No Authentication
1 - Reserved
2 - Reserved
5.3.6.1. Authentication Type 0 - No Authentication
The use of this authentication type means that VRRP protocol
exchanges are not authenticated. The contents of the Authentication
Data field should be set to zero on transmission and ignored on
reception.
5.3.6.2. Authentication Type 1 - Reserved
This authentication type is reserved to maintain backwards
compatibility with RFC 2338.
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5.3.6.3. Authentication Type 2 - Reserved
This authentication type is reserved to maintain backwards
compatibility with RFC 2338.
5.3.7. Advertisement Interval (Adver Int)
The Advertisement interval indicates the time interval (in seconds)
between ADVERTISEMENTS. The default is 1 second. This field is used
for troubleshooting misconfigured routers.
5.3.8. Checksum
The checksum field is used to detect data corruption in the VRRP
message.
The checksum is the 16-bit one's complement of the one's complement
sum of the entire VRRP message starting with the version field. For
computing the checksum, the checksum field is set to zero. See RFC
1071 for more detail [CKSM].
5.3.9. IP Address(es)
One or more IP addresses that are associated with the virtual router.
The number of addresses included is specified in the "Count IP Addrs"
field. These fields are used for troubleshooting misconfigured
routers.
5.3.10. Authentication Data
The authentication string is currently only used to maintain
backwards compatibility with RFC 2338. It SHOULD be set to zero on
transmission and ignored on reception.
6. Protocol State Machine
6.1. Parameters per Virtual Router
VRID Virtual Router Identifier. Configurable item
in the range 1-255 (decimal). There is no
default.
Priority Priority value to be used by this VRRP router
in Master election for this virtual router.
The value of 255 (decimal) is reserved for
the router that owns the IP addresses
associated with the virtual router. The
value of 0 (zero) is reserved for Master
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router to indicate it is releasing
responsibility for the virtual router. The
range 1-254 (decimal) is available for VRRP
routers backing up the virtual router. The
default value is 100 (decimal).
IP_Addresses One or more IP addresses associated with this
virtual router. Configured item. No
default.
Advertisement_Interval Time interval between ADVERTISEMENTS
(seconds). Default is 1 second.
Skew_Time Time to skew Master_Down_Interval in seconds.
Calculated as:
( (256 - Priority) / 256 )
Master_Down_Interval Time interval for Backup to declare Master
down (seconds). Calculated as:
(3 * Advertisement_Interval) + Skew_time
Preempt_Mode Controls whether a higher priority Backup
router preempts a lower priority Master.
Values are True to allow preemption and False
to prohibit preemption. Default is True.
Note: Exception is that the router that owns
the IP address(es) associated with the
virtual router always preempts independent of
the setting of this flag.
Authentication_Type Type of authentication being used. Values
are defined in section 5.3.6.
Authentication_Data Authentication data specific to the
Authentication_Type being used.
6.2. Timers
Master_Down_Timer Timer that fires when ADVERTISEMENT has not
been heard for Master_Down_Interval.
Adver_Timer Timer that fires to trigger sending of
ADVERTISEMENT based on
Advertisement_Interval.
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6.3. State Transition Diagram
+---------------+
+--------->| |<-------------+
| | Initialize | |
| +------| |----------+ |
| | +---------------+ | |
| | | |
| V V |
+---------------+ +---------------+
| |---------------------->| |
| Master | | Backup |
| |<----------------------| |
+---------------+ +---------------+
6.4. State Descriptions
In the state descriptions below, the state names are identified by
{state-name}, and the packets are identified by all upper case
characters.
A VRRP router implements an instance of the state machine for each
virtual router election it is participating in.
6.4.1. Initialize
The purpose of this state is to wait for a Startup event. If a
Startup event is received, then:
- If the Priority = 255 (i.e., the router owns the IP address(es)
associated with the virtual router)
o Send an ADVERTISEMENT
o Broadcast a gratuitous ARP request containing the virtual
router MAC address for each IP address associated with the
virtual router.
o Set the Adver_Timer to Advertisement_Interval
o Transition to the {Master} state
else
o Set the Master_Down_Timer to Master_Down_Interval
o Transition to the {Backup} state
endif
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RFC 3768 VRRP April 2004
6.4.2. Backup
The purpose of the {Backup} state is to monitor the availability and
state of the Master Router.
While in this state, a VRRP router MUST do the following:
- MUST NOT respond to ARP requests for the IP address(s) associated
with the virtual router.
- MUST discard packets with a destination link layer MAC address
equal to the virtual router MAC address.
- MUST NOT accept packets addressed to the IP address(es) associated
with the virtual router.
- If a Shutdown event is received, then:
o Cancel the Master_Down_Timer
o Transition to the {Initialize} state
endif
- If the Master_Down_Timer fires, then:
o Send an ADVERTISEMENT
o Broadcast a gratuitous ARP request containing the virtual
router MAC address for each IP address associated with the
virtual router
o Set the Adver_Timer to Advertisement_Interval
o Transition to the {Master} state
endif
- If an ADVERTISEMENT is received, then:
If the Priority in the ADVERTISEMENT is Zero, then:
o Set the Master_Down_Timer to Skew_Time
else:
If Preempt_Mode is False, or If the Priority in the
ADVERTISEMENT is greater than or equal to the local
Priority, then:
o Reset the Master_Down_Timer to Master_Down_Interval
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else:
o Discard the ADVERTISEMENT
endif
endif
endif
6.4.3. Master
While in the {Master} state the router functions as the forwarding
router for the IP address(es) associated with the virtual router.
While in this state, a VRRP router MUST do the following:
- MUST respond to ARP requests for the IP address(es) associated
with the virtual router.
- MUST forward packets with a destination link layer MAC address
equal to the virtual router MAC address.
- MUST NOT accept packets addressed to the IP address(es) associated
with the virtual router if it is not the IP address owner.
- MUST accept packets addressed to the IP address(es) associated
with the virtual router if it is the IP address owner.
- If a Shutdown event is received, then:
o Cancel the Adver_Timer
o Send an ADVERTISEMENT with Priority = 0
o Transition to the {Initialize} state
endif
- If the Adver_Timer fires, then:
o Send an ADVERTISEMENT o Reset the Adver_Timer to
Advertisement_Interval
endif
- If an ADVERTISEMENT is received, then:
If the Priority in the ADVERTISEMENT is Zero, then:
o Send an ADVERTISEMENT
o Reset the Adver_Timer to Advertisement_Interval
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else:
If the Priority in the ADVERTISEMENT is greater than the
local Priority,
or
If the Priority in the ADVERTISEMENT is equal to the local
Priority and the primary IP Address of the sender is greater
than the local primary IP Address, then:
o Cancel Adver_Timer
o Set Master_Down_Timer to Master_Down_Interval
o Transition to the {Backup} state
else:
o Discard ADVERTISEMENT
endif
endif
endif
7. Sending and Receiving VRRP Packets
7.1. Receiving VRRP Packets
Performed the following functions when a VRRP packet is received:
- MUST verify that the IP TTL is 255.
- MUST verify the VRRP version is 2.
- MUST verify that the received packet contains the complete VRRP
packet (including fixed fields, IP Address(es), and Authentication
Data).
- MUST verify the VRRP checksum.
- MUST verify that the VRID is configured on the receiving interface
and the local router is not the IP Address owner (Priority equals
255 (decimal)).
- MUST verify that the Auth Type matches the locally configured
authentication method for the virtual router and perform that
authentication method.
If any one of the above checks fails, the receiver MUST discard the
packet, SHOULD log the event and MAY indicate via network management
that an error occurred.
- MAY verify that "Count IP Addrs" and the list of IP Address
matches the IP_Addresses configured for the VRID
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If the above check fails, the receiver SHOULD log the event and MAY
indicate via network management that a misconfiguration was detected.
If the packet was not generated by the address owner (Priority does
not equal 255 (decimal)), the receiver MUST drop the packet,
otherwise continue processing.
- MUST verify that the Adver Interval in the packet is the same as
the locally configured for this virtual router
If the above check fails, the receiver MUST discard the packet,
SHOULD log the event and MAY indicate via network management that a
misconfiguration was detected.
7.2. Transmitting VRRP Packets
The following operations MUST be performed when transmitting a VRRP
packet.
- Fill in the VRRP packet fields with the appropriate virtual router
configuration state
- Compute the VRRP checksum
- Set the source MAC address to Virtual Router MAC Address
- Set the source IP address to interface primary IP address
- Set the IP protocol to VRRP
- Send the VRRP packet to the VRRP IP multicast group
Note: VRRP packets are transmitted with the virtual router MAC
address as the source MAC address to ensure that learning bridges
correctly determine the LAN segment the virtual router is attached
to.
7.3. Virtual Router MAC Address
The virtual router MAC address associated with a virtual router is an
IEEE 802 MAC Address in the following format:
00-00-5E-00-01-{VRID} (in hex in internet standard bit-order)
The first three octets are derived from the IANA's OUI. The next two
octets (00-01) indicate the address block assigned to the VRRP
protocol. {VRID} is the VRRP Virtual Router Identifier. This
mapping provides for up to 255 VRRP routers on a network.
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8. Operational Issues
8.1. ICMP Redirects
ICMP Redirects may be used normally when VRRP is running between a
group of routers. This allows VRRP to be used in environments where
the topology is not symmetric.
The IP source address of an ICMP redirect should be the address the
end host used when making its next hop routing decision. If a VRRP
router is acting as Master for virtual router(s) containing addresses
it does not own, then it must determine which virtual router the
packet was sent to when selecting the redirect source address. One
method to deduce the virtual router used is to examine the
destination MAC address in the packet that triggered the redirect.
It may be useful to disable Redirects for specific cases where VRRP
is being used to load share traffic between a number of routers in a
symmetric topology.
8.2. Host ARP Requests
When a host sends an ARP request for one of the virtual router IP
addresses, the Master virtual router MUST respond to the ARP request
with the virtual MAC address for the virtual router. The Master
virtual router MUST NOT respond with its physical MAC address. This
allows the client to always use the same MAC address regardless of
the current Master router.
When a VRRP router restarts or boots, it SHOULD not send any ARP
messages with its physical MAC address for the IP address it owns, it
should only send ARP messages that include Virtual MAC addresses.
This may entail:
- When configuring an interface, VRRP routers should broadcast a
gratuitous ARP request containing the virtual router MAC address
for each IP address on that interface.
- At system boot, when initializing interfaces for VRRP operation;
delay gratuitous ARP requests and ARP responses until both the IP
address and the virtual router MAC address are configured.
8.3. Proxy ARP
If Proxy ARP is to be used on a VRRP router, then the VRRP router
must advertise the Virtual Router MAC address in the Proxy ARP
message. Doing otherwise could cause hosts to learn the real MAC
address of the VRRP router.
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8.4. Potential Forwarding Loop
A VRRP router SHOULD not forward packets addressed to the IP
Address(es) it becomes Master for if it is not the owner. Forwarding
these packets would result in unnecessary traffic. Also in the case
of LANs that receive packets they transmit (e.g., token ring) this
can result in a forwarding loop that is only terminated when the IP
TTL expires.
One such mechanism for VRRP routers is to add/delete a reject host
route for each adopted IP address when transitioning to/from MASTER
state.
9. Operation over FDDI, Token Ring, and ATM LANE
9.1. Operation over FDDI
FDDI interfaces remove from the FDDI ring frames that have a source
MAC address matching the device's hardware address. Under some
conditions, such as router isolations, ring failures, protocol
transitions, etc., VRRP may cause there to be more than one Master
router. If a Master router installs the virtual router MAC address
as the hardware address on a FDDI device, then other Masters'
ADVERTISEMENTS will be removed from the ring during the Master
convergence, and convergence will fail.
To avoid this an implementation SHOULD configure the virtual router
MAC address by adding a unicast MAC filter in the FDDI device, rather
than changing its hardware MAC address. This will prevent a Master
router from removing any ADVERTISEMENTS it did not originate.
9.2. Operation over Token Ring
Token ring has several characteristics that make running VRRP
difficult. These include:
- In order to switch to a new master located on a different bridge
token ring segment from the previous master when using source
route bridges, a mechanism is required to update cached source
route information.
- No general multicast mechanism supported across old and new token
ring adapter implementations. While many newer token ring
adapters support group addresses, token ring functional address
support is the only generally available multicast mechanism. Due
to the limited number of token ring functional addresses these may
collide with other usage of the same token ring functional
addresses.
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Due to these difficulties, the preferred mode of operation over token
ring will be to use a token ring functional address for the VRID
virtual MAC address. Token ring functional addresses have the two
high order bits in the first MAC address octet set to B'1'. They
range from 03-00-00-00-00-80 to 03-00-02-00-00-00 (canonical format).
However, unlike multicast addresses, there is only one unique
functional address per bit position. The functional addresses
03-00-00-10-00-00 through 03-00-02-00-00-00 are reserved by the Token
Ring Architecture [TKARCH] for user-defined applications. However,
since there are only 12 user-defined token ring functional addresses,
there may be other non-IP protocols using the same functional
address. Since the Novell IPX [IPX] protocol uses the
03-00-00-10-00-00 functional address, operation of VRRP over token
ring will avoid use of this functional address. In general, token
ring VRRP users will be responsible for resolution of other user-
defined token ring functional address conflicts.
VRIDs are mapped directly to token ring functional addresses. In
order to decrease the likelihood of functional address conflicts,
allocation will begin with the largest functional address. Most
non-IP protocols use the first or first couple user-defined
functional addresses and it is expected that VRRP users will choose
VRIDs sequentially starting with 1.
VRID Token Ring Functional Address
---- -----------------------------
1 03-00-02-00-00-00
2 03-00-04-00-00-00
3 03-00-08-00-00-00
4 03-00-10-00-00-00
5 03-00-20-00-00-00
6 03-00-40-00-00-00
7 03-00-80-00-00-00
8 03-00-00-01-00-00
9 03-00-00-02-00-00
10 03-00-00-04-00-00
11 03-00-00-08-00-00
Or more succinctly, octets 3 and 4 of the functional address are
equal to (0x4000 >> (VRID - 1)) in non-canonical format.
Since a functional address cannot be used as a MAC level source
address, the real MAC address is used as the MAC source address in
VRRP advertisements. This is not a problem for bridges since packets
addressed to functional addresses will be sent on the spanning-tree
explorer path [802.1D].
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The functional address mode of operation MUST be implemented by
routers supporting VRRP on token ring.
Additionally, routers MAY support unicast mode of operation to take
advantage of newer token ring adapter implementations that support
non-promiscuous reception for multiple unicast MAC addresses and to
avoid both the multicast traffic and usage conflicts associated with
the use of token ring functional addresses. Unicast mode uses the
same mapping of VRIDs to virtual MAC addresses as Ethernet. However,
one important difference exists. ARP request/reply packets contain
the virtual MAC address as the source MAC address. The reason for
this is that some token ring driver implementations keep a cache of
MAC address/source routing information independent of the ARP cache.
Hence, these implementations need to receive a packet with the
virtual MAC address as the source address in order to transmit to
that MAC address in a source-route bridged network.
Unicast mode on token ring has one limitation that should be
considered. If there are VRID routers on different source-route
bridge segments and there are host implementations that keep their
source-route information in the ARP cache and do not listen to
gratuitous ARPs, these hosts will not update their ARP source-route
information correctly when a switch-over occurs. The only possible
solution is to put all routers with the same VRID on the same
source-bridge segment and use techniques to prevent that bridge
segment from being a single point of failure. These techniques are
beyond the scope this document.
For both the multicast and unicast mode of operation, VRRP
advertisements sent to 224.0.0.18 should be encapsulated as described
in [RFC1469].
9.3. Operation over ATM LANE
Operation of VRRP over ATM LANE on routers with ATM LANE interfaces
and/or routers behind proxy LEC's are beyond the scope of this
document.
10. Security Considerations
VRRP does not currently include any type of authentication. Earlier
versions of the VRRP specification included several types of
authentication ranging from none to strong. Operational experience
and further analysis determined that these did not provide any real
measure of security. Due to the nature of the VRRP protocol, even if
VRRP messages are cryptographically protected, it does not prevent
hostile routers from behaving as if they are a VRRP master, creating
multiple masters. Authentication of VRRP messages could have
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RFC 3768 VRRP April 2004
prevented a hostile router from causing all properly functioning
routers from going into backup state. However, having multiple
masters can cause as much disruption as no routers, which
authentication cannot prevent. Also, even if a hostile router could
not disrupt VRRP, it can disrupt ARP and create the same effect as
having all routers go into backup.
It should be noted that these attacks are not worse and are a subset
of the attacks that any node attached to a LAN can do independently
of VRRP. The kind of attacks a malicious node on a LAN can do
include promiscuously receiving packets for any routers MAC address,
sending packets with the routers MAC address as the source MAC
addresses in the L2 header to tell the L2 switches to send packets
addressed to the router to the malicious node instead of the router,
send redirects to tell the hosts to send their traffic somewhere
else, send unsolicited ARP replies, answer ARP requests, etc., etc.
All of this can be done independently of implementing VRRP. VRRP
does not add to these vulnerabilities.
Independent of any authentication type VRRP includes a mechanism
(setting TTL=255, checking on receipt) that protects against VRRP
packets being injected from another remote network. This limits most
vulnerabilities to local attacks.
VRRP does not provide any confidentiality. Confidentiality is not
necessary for the correct operation of VRRP and there is no
information in the VRRP messages that must be kept secret from other
nodes on the LAN.
11. Acknowledgements
The authors would like to thank Glen Zorn, and Michael Lane, Clark
Bremer, Hal Peterson, Tony Li, Barbara Denny, Joel Halpern, Steve
Bellovin, Thomas Narten, Rob Montgomery, Rob Coltun, Radia Perlman,
Russ Housley, Harald Alvestrand, Steve Bellovin, Ned Freed, Ted
Hardie, Russ Housley, Bert Wijnen, Bill Fenner, and Alex Zinin for
their comments and suggestions.
12. References
12.1. Normative References
[802.1D] International Standard ISO/IEC 10038: 1993, ANSI/IEEE Std
802.1D, 1993 edition.
[CKSM] Braden, R., Borman, D. and C. Partridge, "Computing the
Internet checksum", RFC 1071, September 1988.
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RFC 3768 VRRP April 2004
[HSRP] Li, T., Cole, B., Morton, P. and D. Li, "Cisco Hot Standby
Router Protocol (HSRP)", RFC 2281, March 1998.
[IPSTB] Higginson, P. and M. Shand, "Development of Router Clusters
to Provide Fast Failover in IP Networks", Digital Technical
Journal, Volume 9 Number 3, Winter 1997.
[IPX] Novell Incorporated., "IPX Router Specification", Version
1.10, October 1992.
[RFC1469] Pusateri, T., "IP Multicast over Token Ring Local Area
Networks", RFC 1469, June 1993.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2338] Knight, S., Weaver, D., Whipple, D., Hinden, R., Mitzel,
D., Hunt, P., Higginson, P., Shand, M. and A. Lindem,
"Virtual Router Redundancy Protocol", RFC 2338, April 1998.
[TKARCH] IBM Token-Ring Network, Architecture Reference, Publication
SC30-3374-02, Third Edition, (September, 1989).
12.2. Informative References
[DISC] Deering, S., Ed., "ICMP Router Discovery Messages", RFC
1256, September 1991.
[DHCP] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[OSPF] Moy, J., "OSPF version 2", STD 54, RFC 2328, April 1998.
[RIP] Malkin, G., "RIP Version 2", STD 56, RFC 2453, November
1998.
13. Changes from RFC 2338
- Moved authors of RFC 2338 to new Contributers section to comply
with RFC editor policy and listed R. Hinden as Editor.
- Removed authentication methods from VRRP. Changes included:
o Removed the values for password and IPSEC based authentication.
The fields and values are retained to keep backwards
compatibility with RFC 2338.
o Removed section on extensible security
o Updated security consideration section to remove discussion of
different authentication methods and added new text explaining
motivation for change and describe vulnerabilities.
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RFC 3768 VRRP April 2004
- Revised the section 4 examples text with a clearer description of
mapping of IP address owner, priorities, etc.
- Clarify the section 7.1 text describing address list validation.
- Corrected text in Preempt_Mode definition.
- Changed authentication to be per Virtual Router instead of per
Interface.
- Added new subsection (9.3) stating that VRRP over ATM LANE is
beyond the scope of this document.
- Clarified text describing received packet length check.
- Clarified text describing received authentication check.
- Clarified text describing VRID verification check.
- Added new subsection (8.4) describing need to not forward packets
for adopted IP addresses.
- Added clarification to the security considerations section.
- Added reference for computing the internet checksum.
- Updated references and author information.
- Various small editorial changes.
14. Editor's Address
Robert Hinden
Nokia
313 Fairchild Drive
Mountain View, CA 94043
US
Phone: +1 650 625-2004
EMail: bob.hinden@nokia.com
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RFC 3768 VRRP April 2004
15. Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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Acknowledgement
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Internet Society.
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