RFC 3768
This document is obsolete. Please refer to RFC 5798.

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.


   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

Hinden                      Standards Track                     [Page 1]

RFC 3768                          VRRP                        April 2004

       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

Hinden                      Standards Track                     [Page 2]

RFC 3768                          VRRP                        April 2004

   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"

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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.

Hinden                      Standards Track                     [Page 3]

RFC 3768                          VRRP                        April 2004

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

   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,

   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.

Hinden                      Standards Track                     [Page 4]

RFC 3768                          VRRP                        April 2004

   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
   -  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.

Hinden                      Standards Track                     [Page 5]

RFC 3768                          VRRP                        April 2004

   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

Hinden                      Standards Track                     [Page 6]

RFC 3768                          VRRP                        April 2004

   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

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.

Hinden                      Standards Track                     [Page 7]

RFC 3768                          VRRP                        April 2004

            +-----------+      +-----------+
            |   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 |
                                    +-----+  +-----+  +--+--+  +--+--+
              ---+---+---+--  =  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.

Hinden                      Standards Track                     [Page 8]

RFC 3768                          VRRP                        April 2004

   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

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 |
                                    +-----+  +-----+  +--+--+  +--+--+
              ---+---+---+--  =  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.

Hinden                      Standards Track                     [Page 9]

RFC 3768                          VRRP                        April 2004

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

Hinden                      Standards Track                    [Page 10]

RFC 3768                          VRRP                        April 2004

5.2.2.  Destination Address

   The IP multicast address as assigned by the IANA for VRRP is:

   This is a link local scope multicast address.  Routers MUST NOT
   forward a datagram with this destination address regardless of its

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

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.

Hinden                      Standards Track                    [Page 11]

RFC 3768                          VRRP                        April 2004

   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

   The authentication methods currently defined are:

      0 - No Authentication
      1 - Reserved
      2 - Reserved  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.  Authentication Type 1 - Reserved

   This authentication type is reserved to maintain backwards
   compatibility with RFC 2338.

Hinden                      Standards Track                    [Page 12]

RFC 3768                          VRRP                        April 2004  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

   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

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

   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

Hinden                      Standards Track                    [Page 13]

RFC 3768                          VRRP                        April 2004

                           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

   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

Hinden                      Standards Track                    [Page 14]

RFC 3768                          VRRP                        April 2004

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

   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


      o  Set the Master_Down_Timer to Master_Down_Interval
      o  Transition to the {Backup} state


Hinden                      Standards Track                    [Page 15]

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


   -  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


   -  If an ADVERTISEMENT is received, then:

      If the Priority in the ADVERTISEMENT is Zero, then:

      o  Set the Master_Down_Timer to Skew_Time


            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

Hinden                      Standards Track                    [Page 16]

RFC 3768                          VRRP                        April 2004


             o Discard the ADVERTISEMENT


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


      -  If the Adver_Timer fires, then:

      o  Send an ADVERTISEMENT o  Reset the Adver_Timer to


      -  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

Hinden                      Standards Track                    [Page 17]

RFC 3768                          VRRP                        April 2004


            If the Priority in the ADVERTISEMENT is greater than the
            local Priority,
            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


             o Discard ADVERTISEMENT


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
   -  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

Hinden                      Standards Track                    [Page 18]

RFC 3768                          VRRP                        April 2004

   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

   - 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

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.

Hinden                      Standards Track                    [Page 19]

RFC 3768                          VRRP                        April 2004

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.

Hinden                      Standards Track                    [Page 20]

RFC 3768                          VRRP                        April 2004

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

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

Hinden                      Standards Track                    [Page 21]

RFC 3768                          VRRP                        April 2004

   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].

Hinden                      Standards Track                    [Page 22]

RFC 3768                          VRRP                        April 2004

   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 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

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

Hinden                      Standards Track                    [Page 23]

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.

Hinden                      Standards Track                    [Page 24]

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

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.

Hinden                      Standards Track                    [Page 25]

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
   -  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
   313 Fairchild Drive
   Mountain View, CA 94043

   Phone: +1 650 625-2004
   EMail: bob.hinden@nokia.com

Hinden                      Standards Track                    [Page 26]

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

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed
   to pertain to the implementation or use of the technology
   described in this document or the extent to which any license
   under such rights might or might not be available; nor does it
   represent that it has made any independent effort to identify any
   such rights.  Information on the procedures with respect to
   rights in RFC documents can be found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use
   of such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository
   at http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention
   any copyrights, patents or patent applications, or other
   proprietary rights that may cover technology that may be required
   to implement this standard.  Please address the information to the
   IETF at ietf-ipr@ietf.org.


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

Hinden                      Standards Track                    [Page 27]