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rfc:rfc5798

Internet Engineering Task Force (IETF) S. Nadas, Ed. Request for Comments: 5798 Ericsson Obsoletes: 3768 March 2010 Category: Standards Track ISSN: 2070-1721

Virtual Router Redundancy Protocol (VRRP) Version 3 for IPv4 and IPv6

Abstract

 This memo defines the Virtual Router Redundancy Protocol (VRRP) for
 IPv4 and IPv6.  It is version three (3) of the protocol, and it is
 based on VRRP (version 2) for IPv4 that is defined in RFC 3768 and in
 "Virtual Router Redundancy Protocol for IPv6".  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 IPv4 or IPv6 address(es) associated with a virtual
 router is called the Master, and it forwards packets sent to these
 IPv4 or IPv6 addresses.  VRRP Master routers are configured with
 virtual IPv4 or IPv6 addresses, and VRRP Backup routers infer the
 address family of the virtual addresses being carried based on the
 transport protocol.  Within a VRRP router, the virtual routers in
 each of the IPv4 and IPv6 address families are a domain unto
 themselves and do not overlap.  The election process provides dynamic
 failover in the forwarding responsibility should the Master become
 unavailable.  For IPv4, 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.  For
 IPv6, the advantage gained from using VRRP for IPv6 is a quicker
 switchover to Backup routers than can be obtained with standard IPv6
 Neighbor Discovery mechanisms.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc5798.

Nadas Standards Track [Page 1] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

Copyright Notice

 Copyright (c) 2010 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Table of Contents

 1. Introduction ....................................................4
    1.1. A Note on Terminology ......................................4
    1.2. IPv4 .......................................................5
    1.3. IPv6 .......................................................6
    1.4. Requirements Language ......................................6
    1.5. Scope ......................................................7
    1.6. Definitions ................................................7
 2. Required Features ...............................................8
    2.1. IPvX Address Backup ........................................8
    2.2. Preferred Path Indication ..................................8
    2.3. Minimization of Unnecessary Service Disruptions ............9
    2.4. Efficient Operation over Extended LANs .....................9
    2.5. Sub-Second Operation for IPv4 and IPv6 .....................9
 3. VRRP Overview ..................................................10
 4. Sample Configurations ..........................................11
    4.1. Sample Configuration 1 ....................................11
    4.2. Sample Configuration 2 ....................................13

Nadas Standards Track [Page 2] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

 5. Protocol .......................................................14
    5.1. VRRP Packet Format ........................................15
         5.1.1. IPv4 Field Descriptions ............................15
                5.1.1.1. Source Address ............................15
                5.1.1.2. Destination Address .......................15
                5.1.1.3. TTL .......................................16
                5.1.1.4. Protocol ..................................16
         5.1.2. IPv6 Field Descriptions ............................16
                5.1.2.1. Source Address ............................16
                5.1.2.2. Destination Address .......................16
                5.1.2.3. Hop Limit .................................16
                5.1.2.4. Next Header ...............................16
    5.2. VRRP Field Descriptions ...................................16
         5.2.1. Version ............................................16
         5.2.2. Type ...............................................17
         5.2.3. Virtual Rtr ID (VRID) ..............................17
         5.2.4. Priority ...........................................17
         5.2.5. Count IPvX Addr ....................................17
         5.2.6. Rsvd ...............................................17
         5.2.7. Maximum Advertisement Interval (Max Adver Int) .....17
         5.2.8. Checksum ...........................................18
         5.2.9. IPvX Address(es) ...................................18
 6. Protocol State Machine .........................................18
    6.1. Parameters Per Virtual Router .............................18
    6.2. Timers ....................................................20
    6.3. State Transition Diagram ..................................21
    6.4. State Descriptions ........................................21
         6.4.1. Initialize .........................................21
         6.4.2. Backup .............................................22
         6.4.3. Master .............................................24
 7. Sending and Receiving VRRP Packets .............................26
    7.1. Receiving VRRP Packets ....................................26
    7.2. Transmitting VRRP Packets .................................27
    7.3. Virtual Router MAC Address ................................28
    7.4. IPv6 Interface Identifiers ................................28
 8. Operational Issues .............................................29
    8.1. IPv4 ......................................................29
         8.1.1. ICMP Redirects .....................................29
         8.1.2. Host ARP Requests ..................................29
         8.1.3. Proxy ARP ..........................................30
    8.2. IPv6 ......................................................30
         8.2.1. ICMPv6 Redirects ...................................30
         8.2.2. ND Neighbor Solicitation ...........................30
         8.2.3. Router Advertisements ..............................31
    8.3. IPvX ......................................................31
         8.3.1. Potential Forwarding Loop ..........................31
         8.3.2. Recommendations Regarding Setting Priority Values ..32

Nadas Standards Track [Page 3] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

    8.4. VRRPv3 and VRRPv2 Interoperation ..........................32
         8.4.1. Assumptions ........................................32
         8.4.2. VRRPv3 Support of VRRPv2 ...........................32
         8.4.3. VRRPv3 Support of VRRPv2 Considerations ............33
                8.4.3.1. Slow, High-Priority Masters ...............33
                8.4.3.2. Overwhelming VRRPv2 Backups ...............33
 9. Security Considerations ........................................33
 10. Contributors and Acknowledgments ..............................34
 11. IANA Considerations ...........................................35
 12. References ....................................................35
    12.1. Normative References .....................................35
    12.2. Informative References ...................................36
 Appendix A. Operation over FDDI, Token Ring, and ATM LANE .........38
    A.1. Operation over FDDI .......................................38
    A.2. Operation over Token Ring .................................38
    A.3. Operation over ATM LANE ...................................40

1. Introduction

 This memo defines the Virtual Router Redundancy Protocol (VRRP) for
 IPv4 and IPv6.  It is version three (3) of the protocol.  It is based
 on VRRP (version 2) for IPv4 that is defined in [RFC3768] and in
 [VRRP-IPv6].  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 IPv4 or IPv6
 address(es) associated with a virtual router is called the Master,
 and it forwards packets sent to these IPv4 or IPv6 addresses.  VRRP
 Master routers are configured with virtual IPv4 or IPv6 addresses,
 and VRRP Backup routers infer the address family of the virtual
 addresses being carried based on the transport protocol.  Within a
 VRRP router, the virtual routers in each of the IPv4 and IPv6 address
 families are a domain unto themselves and do not overlap.  The
 election process provides dynamic failover in the forwarding
 responsibility should the Master become unavailable.
 VRRP provides a function similar to the proprietary protocols "Hot
 Standby Router Protocol (HSRP)" [RFC2281] and "IP Standby Protocol"
 [IPSTB].

1.1. A Note on Terminology

 This document discusses both IPv4 and IPv6 operation, and with
 respect to the VRRP protocol, many of the descriptions and procedures
 are common.  In this document, it would be less verbose to be able to
 refer to "IP" to mean either "IPv4 or IPv6".  However, historically,
 the term "IP" usually refers to IPv4.  For this reason, in this
 specification, the term "IPvX" (where X is 4 or 6) is introduced to
 mean either "IPv4" or "IPv6".  In this text, where the IP version

Nadas Standards Track [Page 4] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

 matters, the appropriate term is used and the use of the term "IP" is
 avoided.

1.2. IPv4

 There are a number of methods that an IPv4 end-host can use to
 determine its first-hop router towards a particular IPv4 destination.
 These include running (or snooping) a dynamic routing protocol such
 as Routing Information Protocol [RFC2453] or OSPF version 2
 [RFC2328], running an ICMP router discovery client [RFC1256], 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
 of large numbers of hosts.  This can result in a significant delay in
 the detection of a lost (i.e., dead) neighbor; such a delay 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 IPv4 implementation.  This mode of
 operation is likely to persist as dynamic host configuration
 protocols [RFC2131] are deployed, which typically provide
 configuration for an end-host IPv4 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 IPv4
 address(es) associated with a virtual router is called the Master and
 forwards packets sent to these IPv4 addresses.  The election process
 provides dynamic failover in the forwarding responsibility should the
 Master become unavailable.  Any of the virtual router's IPv4
 addresses on a LAN can then be used as the default first hop

Nadas Standards Track [Page 5] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

 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.

1.3. IPv6

 IPv6 hosts on a LAN will usually learn about one or more default
 routers by receiving Router Advertisements sent using the IPv6
 Neighbor Discovery (ND) protocol [RFC4861].  The Router
 Advertisements are multicast periodically at a rate that the hosts
 will learn about the default routers in a few minutes.  They are not
 sent frequently enough to rely on the absence of the Router
 Advertisement to detect router failures.
 Neighbor Discovery (ND) includes a mechanism called Neighbor
 Unreachability Detection to detect the failure of a neighbor node
 (router or host) or the forwarding path to a neighbor.  This is done
 by sending unicast ND Neighbor Solicitation messages to the neighbor
 node.  To reduce the overhead of sending Neighbor Solicitations, they
 are only sent to neighbors to which the node is actively sending
 traffic and only after there has been no positive indication that the
 router is up for a period of time.  Using the default parameters in
 ND, it will take a host about 38 seconds to learn that a router is
 unreachable before it will switch to another default router.  This
 delay would be very noticeable to users and cause some transport
 protocol implementations to time out.
 While the ND unreachability detection could be made quicker by
 changing the parameters to be more aggressive (note that the current
 lower limit for this is 5 seconds), this would have the downside of
 significantly increasing the overhead of ND traffic, especially when
 there are many hosts all trying to determine the reachability of one
 of more routers.
 The Virtual Router Redundancy Protocol for IPv6 provides a much
 faster switchover to an alternate default router than can be obtained
 using standard ND procedures.  Using VRRP, a Backup router can take
 over for a failed default router in around three seconds (using VRRP
 default parameters).  This is done without any interaction with the
 hosts and a minimum amount of VRRP traffic.

1.4. Requirements Language

 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].

Nadas Standards Track [Page 6] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

1.5. 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.

1.6. Definitions

 VRRP Router             A router running the Virtual Router
                         Redundancy Protocol.  It may participate as
                         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 either a set of associated
                         IPv4 addresses or a set of associated IPv6
                         addresses across a common LAN.  A VRRP Router
                         may back up one or more virtual routers.
 IP Address Owner        The VRRP router that has the virtual router's
                         IPvX address(es) as real interface
                         address(es).  This is the router that, when
                         up, will respond to packets addressed to one
                         of these IPvX addresses for ICMP pings, TCP
                         connections, etc.
 Primary IP Address      In IPv4, an IPv4 address selected from the
                         set of real interface addresses.  One
                         possible selection algorithm is to always
                         select the first address.  In IPv4 mode, VRRP
                         advertisements are always sent using the
                         primary IPv4 address as the source of the
                         IPv4 packet.  In IPv6, the link-local address
                         of the interface over which the packet is
                         transmitted is used.
 Virtual Router Master   The VRRP router that is assuming the
                         responsibility of forwarding packets sent to
                         the IPvX address(es) associated with the
                         virtual router, answering ARP requests

Nadas Standards Track [Page 7] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

                         for the IPv4 address(es), and answering ND
                         requests for the IPv6 address(es).  Note that
                         if the IPvX address owner is available, then
                         it will always become the Master.
 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. IPvX Address Backup

 Backup of an IPvX address or addresses is the primary function of
 VRRP.  While providing election of a Virtual Router Master and the
 additional functionality described below, the protocol should
 strive to:
 o  Minimize the duration of black holes.
 o  Minimize the steady-state bandwidth overhead and processing
    complexity.
 o  Function over a wide variety of multiaccess LAN technologies
    capable of supporting IPvX traffic.
 o  Allow multiple virtual routers on a network for load balancing.
 o  Support multiple logical IPvX 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.

Nadas Standards Track [Page 8] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

2.3. Minimization of Unnecessary Service Disruptions

 Once Master election has been performed, 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.
 Some environments may find it beneficial to avoid the state
 transition triggered when a router that is preferred over the current
 Master becomes available.  It may be useful to support an override of
 the immediate convergence to the preferred path.

2.4. Efficient Operation over Extended LANs

 Sending IPvX packets (that is, sending either IPv4 or IPv6) on a
 multiaccess LAN requires mapping from an IPvX 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 address as the source in a packet sent
 by the Master to trigger station learning; 2) trigger a message
 immediately after transitioning to the Master to update the station
 learning; and 3) trigger periodic messages from the Master to
 maintain the station learning cache.

2.5. Sub-Second Operation for IPv4 and IPv6

 Sub-second detection of Master VRRP router failure is needed in both
 IPv4 and IPv6 environments.  Earlier work proposed that sub-second
 operation was for IPv6; this specification leverages that earlier
 approach for IPv4 and IPv6.
 One possible problematic scenario when using small
 VRRP_Advertisement_Intervals may occur when a router is delivering
 more packets onto the LAN than can be accommodated, and so a queue
 builds up in the router.  It is possible that packets being
 transmitted onto the VRRP-protected LAN could see larger queueing
 delay than the smallest VRRP Advertisement_Interval.  In this case,
 the Master_Down_Interval will be small enough so that normal queuing
 delays might cause a VRRP Backup to conclude that the Master is down,
 and therefore promote itself to Master.  Very shortly afterwards, the
 delayed VRRP packets from the Master cause a switch back to Backup
 status.  Furthermore, this process can repeat many times per second,

Nadas Standards Track [Page 9] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

 causing significant disruption to traffic.  To mitigate this problem,
 priority forwarding of VRRP packets should be considered.  It should
 be possible for a VRRP Master to observe that this situation is
 occurring frequently and at least log the problem.

3. VRRP Overview

 VRRP specifies an election protocol to provide the virtual router
 function described earlier.  All protocol messaging is performed
 using either IPv4 or IPv6 multicast datagrams; thus, the protocol can
 operate over a variety of multiaccess LAN technologies supporting
 IPvX multicast.  Each link of a 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 either IPv4 or IPv6 address(es).  A VRRP router may
 associate a virtual router with its real address on an interface.
 The scope of each virtual router is restricted to a single LAN.  A
 VRRP router may be configured with additional virtual router mappings
 and priority for virtual routers it is willing to back up.  The
 mapping between the VRID and its IPvX address(es) must be coordinated
 among all VRRP routers on a LAN.
 There is no restriction against reusing a VRID with a different
 address mapping on different LANs, nor is there a restriction against
 using the same VRID number for a set of IPv4 addresses and a set of
 IPv6 addresses; however, these are two different virtual routers.
 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
 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

Nadas Standards Track [Page 10] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

 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.
      +-----------+ +-----------+
      |   Rtr1    | |   Rtr2    |
      |(MR VRID=1)| |(BR VRID=1)|
      |           | |           |

VRID=1 +———–+ +———–+ IPvX A———>* *←——–IPvX B

              |            |
              |            |

—————-+————+—–+———-+———-+———-+–

                                 ^          ^          ^          ^
                                 |          |          |          |

default rtr IPvX addrs——→ (IPvX A) (IPvX A) (IPvX A) (IPvX A)

                                 |          |          |          |
                        IPvX H1->* IpvX H2->* IPvX H3->* IpvX H4->*
                              +--+--+    +--+--+    +--+--+    +--+--+
                              |  H1 |    |  H2 |    |  H3 |    |  H4 |
                              +-----+    +-----+    +--+--+    +--+--+
 Legend:
       --+---+---+-- = Ethernet, Token Ring, or FDDI
                   H = Host computer
                  MR = Master Router
                  BR = Backup Router
                  *  =  IPvX Address; X is 4 everywhere in IPv4 case
                                      X is 6 everywhere in IPv6 case
                  (IPvX) = default router for hosts

Nadas Standards Track [Page 11] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

 Eliminating all mention of VRRP (VRID=1) from the figure above leaves
 it as a typical deployment.
 In the IPv4 case (that is, IPvX is IPv4 everywhere in the figure),
 each router is permanently assigned an IPv4 address on the LAN
 interface (Rtr1 is assigned IPv4 A and Rtr2 is assigned IPv4 B), and
 each host installs a static default route through one of the routers
 (in this example they all use Rtr1's IPv4 A).
 In the IPv6 case (that is, IPvX is IPv6 everywhere in the figure),
 each router has a link-local IPv6 address on the LAN interface (Rtr1
 is assigned IPv6 Link-Local A and Rtr2 is assigned IPv6 Link-
 Local B), and each host learns a default route from Router
 Advertisements through one of the routers (in this example, they all
 use Rtr1's IPv6 Link-Local A).
 Moving to an IPv4 VRRP environment, each router has the exact same
 permanently assigned IPv4 address.  Rtr1 is said to be the IPv4
 address owner of IPv4 A, and Rtr2 is the IPv4 address owner of
 IPv4 B.  A virtual router is then defined by associating a unique
 identifier (the virtual router ID) with the address owned by a
 router.
 Moving to an IPv6 VRRP environment, each router has the exact same
 Link-Local IPv6 address.  Rtr1 is said to be the IPv6 address owner
 of IPv6 A, and Rtr2 is the IPv6 address owner of IPv6 B.  A virtual
 router is then defined by associating a unique identifier (the
 virtual router ID) with the address owned by a router.
 Finally, in both the IPv4 and IPv6 cases, the VRRP protocol manages
 virtual router failover to a Backup router.
 The IPv4 example above shows a virtual router configured to cover the
 IPv4 address owned by Rtr1 (VRID=1, IPv4_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 (the default priority is
 100), since it is not the IPv4 address owner.  If Rtr1 should fail,
 then the VRRP protocol will transition Rtr2 to Master, temporarily
 taking over forwarding responsibility for IPv4 A to provide
 uninterrupted service to the hosts.  When Rtr1 returns to service, it
 will re-assert itself as Master.
 The IPv6 example above shows a virtual router configured to cover the
 IPv6 address owned by Rtr1 (VRID=1, IPv6_Address=A).  When VRRP is
 enabled on Rtr1 for VRID=1, it will assert itself as Master, with
 priority = 255, since it is the IPv6 address owner for the virtual

Nadas Standards Track [Page 12] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

 router IPv6 address.  When VRRP is enabled on Rtr2 for VRID=1, it
 will transition to Backup, with priority = 100 (the default priority
 is 100), since it is not the IPv6 address owner.  If Rtr1 should
 fail, then the VRRP protocol will transition Rtr2 to Master,
 temporarily taking over forwarding responsibility for IPv6 A to
 provide uninterrupted service to the hosts.
 Note that in both cases, in this example IPvX B is not backed up; it
 is only used by Rtr2 as its interface address.  In order to back up
 IPvX 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 splitting their traffic between them.
      +-----------+      +-----------+
      |   Rtr1    |      |   Rtr2    |
      |(MR VRID=1)|      |(BR VRID=1)|
      |(BR VRID=2)|      |(MR VRID=2)|

VRID=1 +———–+ +———–+ VRID=2 IPvX A ——–>* *←——— IPvX B

              |            |
              |            |

—————-+————+—–+———-+———-+———-+–

                                 ^          ^          ^          ^
                                 |          |          |          |

default rtr IPvX addrs —–> (IPvX A) (IPvX A) (IPvX B) (IPvX B)

                                 |          |          |          |
                        IPvX H1->* IpvX H2->* IPvX H3->* IpvX H4->*
                              +--+--+    +--+--+    +--+--+    +--+--+
                              |  H1 |    |  H2 |    |  H3 |    |  H4 |
                              +-----+    +-----+    +--+--+    +--+--+
 Legend:
      ---+---+---+--  =  Ethernet, Token Ring, or FDDI
                   H  =  Host computer
                  MR  =  Master Router
                  BR  =  Backup Router
                   *  =  IPvX Address; X is 4 everywhere in IPv4 case
                                       X is 6 everywhere in IPv6 case
              (IPvX)  =  default router for hosts
 In the IPv4 example above (that is, IPvX is IPv4 everywhere in the
 figure), half of the hosts have configured a static route through
 Rtr1's IPv4 A, and half are using Rtr2's IPv4 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

Nadas Standards Track [Page 13] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

 cover the IPv4 address owned by Rtr2 (VRID=2, IPv4_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.
 In the IPv6 example above (that is, IPvX is IPv6 everywhere in the
 figure), half of the hosts have learned a default route through
 Rtr1's IPv6 A, and half are using Rtr2's IPv6 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 IPv6 address owned by Rtr2 (VRID=2, IPv6_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.
 Note that the details of load balancing are out of scope of this
 document.  However, in a case where the servers need different
 weights, it may not make sense to rely on router advertisements alone
 to balance the host load between the routers.

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
 VRID.
 When VRRP is protecting an IPv4 address, VRRP packets are sent
 encapsulated in IPv4 packets.  They are sent to the IPv4 multicast
 address assigned to VRRP.
 When VRRP is protecting an IPv6 address, VRRP packets are sent
 encapsulated in IPv6 packets.  They are sent to the IPv6 multicast
 address assigned to VRRP.

Nadas Standards Track [Page 14] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                    IPv4 Fields or IPv6 Fields                 |
 ...                                                             ...
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |Version| Type  | Virtual Rtr ID|   Priority    |Count IPvX Addr|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |(rsvd) |     Max Adver Int     |          Checksum             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +                                                               +
  |                       IPvX Address(es)                        |
  +                                                               +
  +                                                               +
  +                                                               +
  +                                                               +
  |                                                               |
  +                                                               +
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.1.1. IPv4 Field Descriptions

5.1.1.1. Source Address

 This is the primary IPv4 address of the interface the packet is being
 sent from.

5.1.1.2. Destination Address

 The IPv4 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.

Nadas Standards Track [Page 15] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

5.1.1.3. 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.

5.1.1.4. Protocol

 The IPv4 protocol number assigned by the IANA for VRRP is 112
 (decimal).

5.1.2. IPv6 Field Descriptions

5.1.2.1. Source Address

 This is the IPv6 link-local address of the interface the packet is
 being sent from.

5.1.2.2. Destination Address

 The IPv6 multicast address assigned by the IANA for VRRP is:
    FF02:0:0:0:0:0:0:12
 This is a link-local scope multicast address.  Routers MUST NOT
 forward a datagram with this destination address, regardless of its
 Hop Limit.

5.1.2.3. Hop Limit

 The Hop Limit MUST be set to 255.  A VRRP router receiving a packet
 with the Hop Limit not equal to 255 MUST discard the packet.

5.1.2.4. Next Header

 The IPv6 Next Header protocol assigned by the IANA for VRRP is 112
 (decimal).

5.2. VRRP Field Descriptions

5.2.1. Version

 The version field specifies the VRRP protocol version of this packet.
 This document defines version 3.

Nadas Standards Track [Page 16] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

5.2.2. 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.

5.2.3. Virtual Rtr ID (VRID)

 The Virtual Rtr ID field identifies the virtual router this packet is
 reporting status for.

5.2.4. 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.
 The priority value for the VRRP router that owns the IPvX address
 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 time out.

5.2.5. Count IPvX Addr

 This is the number of either IPv4 addresses or IPv6 addresses
 contained in this VRRP advertisement.  The minimum value is 1.

5.2.6. Rsvd

 This field MUST be set to zero on transmission and ignored on
 reception.

5.2.7. Maximum Advertisement Interval (Max Adver Int)

 The Maximum Advertisement Interval is a 12-bit field that indicates
 the time interval (in centiseconds) between ADVERTISEMENTS.  The
 default is 100 centiseconds (1 second).

Nadas Standards Track [Page 17] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

 Note that higher-priority Master routers with slower transmission
 rates than their Backup routers are unstable.  This is because low-
 priority nodes configured to faster rates could come online and
 decide they should be Masters before they have heard anything from
 the higher-priority Master with a slower rate.  When this happens, it
 is temporary: once the lower-priority node does hear from the higher-
 priority Master, it will relinquish mastership.

5.2.8. 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 and a
 "pseudo-header" as defined in Section 8.1 of [RFC2460].  The next
 header field in the "pseudo-header" should be set to 112 (decimal)
 for VRRP.  For computing the checksum, the checksum field is set to
 zero.  See RFC1071 for more detail [RFC1071].

5.2.9. IPvX Address(es)

 This refers to one or more IPvX addresses associated with the virtual
 router.  The number of addresses included is specified in the "Count
 IP Addr" field.  These fields are used for troubleshooting
 misconfigured routers.  If more than one address is sent, it is
 recommended that all routers be configured to send these addresses in
 the same order to make it easier to do this comparison.
 For IPv4 addresses, this refers to one or more IPv4 addresses that
 are backed up by the virtual router.
 For IPv6, the first address must be the IPv6 link-local address
 associated with the virtual router.
 This field contains either one or more IPv4 addresses, or one or more
 IPv6 addresses, that is, IPv4 and IPv6 MUST NOT both be carried in
 one IPvX Address field.

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.

Nadas Standards Track [Page 18] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

 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 IPvX address associated with the
                             virtual router.  The value of 0 (zero) is
                             reserved for the Master 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.  Higher
                             values indicate higher priorities.  The
                             default value is 100 (decimal).
 IPv4_Addresses              One or more IPv4 addresses associated
                             with this virtual router.  Configured
                             item with no default.
 IPv6_Addresses              One or more IPv6 addresses associated
                             with this virtual router.  Configured
                             item with no default.  The first address
                             must be the Link-Local address associated
                             with the virtual router.
 Advertisement_Interval      Time interval between ADVERTISEMENTS
                             (centiseconds).  Default is 100
                             centiseconds (1 second).
 Master_Adver_Interval       Advertisement interval contained in
                             ADVERTISEMENTS received from the Master
                             (centiseconds).  This value is saved by
                             virtual routers in the Backup state and
                             used to compute Skew_Time and
                             Master_Down_Interval.  The initial value
                             is the same as Advertisement_Interval.
 Skew_Time                   Time to skew Master_Down_Interval in
                             centiseconds.  Calculated as
                 (((256 - priority) * Master_Adver_Interval) / 256)
 Master_Down_Interval        Time interval for Backup to declare
                             Master down (centiseconds).
                             Calculated as
                             (3 * Master_Adver_Interval) + Skew_time

Nadas Standards Track [Page 19] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

 Preempt_Mode                Controls whether a (starting or
                             restarting) higher-priority Backup router
                             preempts a lower-priority Master router.
                             Values are True to allow preemption and
                             False to prohibit preemption.  Default is
                             True.
                             Note: The exception is that the router
                             that owns the IPvX address associated
                             with the virtual router always preempts,
                             independent of the setting of this flag.
 Accept_Mode                 Controls whether a virtual router in
                             Master state will accept packets
                             addressed to the address owner's IPvX
                             address as its own if it is not the IPvX
                             address owner.  The default is False.
                             Deployments that rely on, for example,
                             pinging the address owner's IPvX address
                             may wish to configure Accept_Mode to
                             True.
                             Note: IPv6 Neighbor Solicitations and
                             Neighbor Advertisements MUST NOT be
                             dropped when Accept_Mode is False.
 Virtual_Router_MAC_Address  The MAC address used for the source MAC
                             address in VRRP advertisements and
                             advertised in ARP responses as the MAC
                             address to use for IP_Addresses.

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.

Nadas Standards Track [Page 20] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

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-uppercase
 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, that is, an
 implementation-defined mechanism that initiates the protocol once it
 has been configured.  The configuration mechanism is out of scope of
 this specification.
 (100) If a Startup event is received, then:
    (105) - If the Priority = 255 (i.e., the router owns the IPvX
    address associated with the virtual router), then:
       (110) + Send an ADVERTISEMENT
       (115) + If the protected IPvX address is an IPv4 address, then:
          (120) * Broadcast a gratuitous ARP request containing the
          virtual router MAC address for each IP address associated
          with the virtual router.
       (125) + else // IPv6
          (130) * For each IPv6 address associated with the virtual
          router, send an unsolicited ND Neighbor Advertisement with

Nadas Standards Track [Page 21] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

          the Router Flag (R) set, the Solicited Flag (S) unset, the
          Override flag (O) set, the target address set to the IPv6
          address of the virtual router, and the target link-layer
          address set to the virtual router MAC address.
       (135) +endif // was protected addr IPv4?
       (140) + Set the Adver_Timer to Advertisement_Interval
       (145) + Transition to the {Master} state
    (150) - else // rtr does not own virt addr
       (155) + Set Master_Adver_Interval to Advertisement_Interval
       (160) + Set the Master_Down_Timer to Master_Down_Interval
       (165) + Transition to the {Backup} state
    (170) -endif // priority was not 255
    (175) endif // startup event was recv

6.4.2. Backup

 The purpose of the {Backup} state is to monitor the availability and
 state of the Master router.
 (300) While in this state, a VRRP router MUST do the following:
    (305) - If the protected IPvX address is an IPv4 address, then:
       (310) + MUST NOT respond to ARP requests for the IPv4
       address(es) associated with the virtual router.
    (315) - else // protected addr is IPv6
       (320) + MUST NOT respond to ND Neighbor Solicitation messages
       for the IPv6 address(es) associated with the virtual router.
       (325) + MUST NOT send ND Router Advertisement messages for the
       virtual router.
    (330) -endif // was protected addr IPv4?
    (335) - MUST discard packets with a destination link-layer MAC
    address equal to the virtual router MAC address.

Nadas Standards Track [Page 22] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

    (340) - MUST NOT accept packets addressed to the IPvX address(es)
    associated with the virtual router.
    (345) - If a Shutdown event is received, then:
       (350) + Cancel the Master_Down_Timer
       (355) + Transition to the {Initialize} state
    (360) -endif // shutdown recv
    (365) - If the Master_Down_Timer fires, then:
       (370) + Send an ADVERTISEMENT
       (375) + If the protected IPvX address is an IPv4 address, then:
          (380) * Broadcast a gratuitous ARP request on that interface
          containing the virtual router MAC address for each IPv4
          address associated with the virtual router.
       (385) + else // ipv6
          (390) * Compute and join the Solicited-Node multicast
          address [RFC4291] for the IPv6 address(es) associated with
          the virtual router.
          (395) * For each IPv6 address associated with the virtual
          router, send an unsolicited ND Neighbor Advertisement with
          the Router Flag (R) set, the Solicited Flag (S) unset, the
          Override flag (O) set, the target address set to the IPv6
          address of the virtual router, and the target link-layer
          address set to the virtual router MAC address.
       (400) +endif // was protected addr ipv4?
       (405) + Set the Adver_Timer to Advertisement_Interval
       (410) + Transition to the {Master} state
    (415) -endif // Master_Down_Timer fired
    (420) - If an ADVERTISEMENT is received, then:
       (425) + If the Priority in the ADVERTISEMENT is zero, then:
          (430) * Set the Master_Down_Timer to Skew_Time

Nadas Standards Track [Page 23] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

       (440) + else // priority non-zero
          (445) * If Preempt_Mode is False, or if the Priority in the
          ADVERTISEMENT is greater than or equal to the local
          Priority, then:
             (450) @ Set Master_Adver_Interval to Adver Interval
             contained in the ADVERTISEMENT
             (455) @ Recompute the Master_Down_Interval
             (460) @ Reset the Master_Down_Timer to
             Master_Down_Interval
          (465) * else // preempt was true or priority was less
             (470) @ Discard the ADVERTISEMENT
          (475) *endif // preempt test
       (480) +endif // was priority zero?
    (485) -endif // was advertisement recv?
 (490) endwhile // Backup state

6.4.3. Master

 While in the {Master} state, the router functions as the forwarding
 router for the IPvX address(es) associated with the virtual router.
 Note that in the Master state, the Preempt_Mode Flag is not
 considered.
 (600) While in this state, a VRRP router MUST do the following:
    (605) - If the protected IPvX address is an IPv4 address, then:
       (610) + MUST respond to ARP requests for the IPv4 address(es)
       associated with the virtual router.
    (615) - else // ipv6
       (620) + MUST be a member of the Solicited-Node multicast
       address for the IPv6 address(es) associated with the virtual
       router.

Nadas Standards Track [Page 24] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

       (625) + MUST respond to ND Neighbor Solicitation message for
       the IPv6 address(es) associated with the virtual router.
       (630) ++ MUST send ND Router Advertisements for the virtual
       router.
       (635) ++ If Accept_Mode is False:  MUST NOT drop IPv6 Neighbor
       Solicitations and Neighbor Advertisements.
    (640) +-endif // ipv4?
    (645) - MUST forward packets with a destination link-layer MAC
    address equal to the virtual router MAC address.
    (650) - MUST accept packets addressed to the IPvX address(es)
    associated with the virtual router if it is the IPvX address owner
    or if Accept_Mode is True.  Otherwise, MUST NOT accept these
    packets.
    (655) - If a Shutdown event is received, then:
       (660) + Cancel the Adver_Timer
       (665) + Send an ADVERTISEMENT with Priority = 0
       (670) + Transition to the {Initialize} state
    (675) -endif // shutdown recv
    (680) - If the Adver_Timer fires, then:
       (685) + Send an ADVERTISEMENT
       (690) + Reset the Adver_Timer to Advertisement_Interval
    (695) -endif // advertisement timer fired
    (700) - If an ADVERTISEMENT is received, then:
       (705) -+ If the Priority in the ADVERTISEMENT is zero, then:
          (710) -* Send an ADVERTISEMENT
          (715) -* Reset the Adver_Timer to Advertisement_Interval
       (720) -+ else // priority was non-zero

Nadas Standards Track [Page 25] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

          (725) -* If the Priority in the ADVERTISEMENT is greater
          than the local Priority,
          (730) -* or
          (735) -* If the Priority in the ADVERTISEMENT is equal to
          the local Priority and the primary IPvX Address of the
          sender is greater than the local primary IPvX Address, then:
             (740) -@ Cancel Adver_Timer
             (745) -@ Set Master_Adver_Interval to Adver Interval
             contained in the ADVERTISEMENT
             (750) -@ Recompute the Skew_Time
             (755) @ Recompute the Master_Down_Interval
             (760) @ Set Master_Down_Timer to Master_Down_Interval
             (765) @ Transition to the {Backup} state
          (770) * else // new Master logic
             (775) @ Discard ADVERTISEMENT
          (780) *endif // new Master detected
       (785) +endif // was priority zero?
    (790) -endif // advert recv
 (795) endwhile // in Master

7. Sending and Receiving VRRP Packets

7.1. Receiving VRRP Packets

 The following functions are performed when a VRRP packet is received:
  1. If the received packet is an IPv4 packet, then:
       + MUST verify that the IPv4 TTL is 255.
  1. else ipv6 recv + MUST verify that the IPv6 Hop Limit is 255. Nadas Standards Track [Page 26] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010 -endif - MUST verify that the VRRP version is 3. - MUST verify that the received packet contains the complete VRRP packet (including fixed fields, and IPvX address). - MUST verify the VRRP checksum. - MUST verify that the VRID is configured on the receiving interface and the local router is not the IPvX address owner (Priority = 255 (decimal)). 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 IPvX Addrs" and the list of IPvX address(es) match the IPvX Address(es) configured for the VRID. If the above check fails, the receiver 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 - If the protected address is an IPv4 address, then: + Set the source MAC address to virtual router MAC Address + Set the source IPv4 address to interface primary IPv4 address - else ipv6
       + Set the source MAC address to virtual router MAC Address
       + Set the source IPv6 address to interface link-local IPv6
       address
  1. endif

Nadas Standards Track [Page 27] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

  1. Set the IPvX protocol to VRRP
  1. Send the VRRP packet to the VRRP IPvX 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:
 IPv4 case: 00-00-5E-00-01-{VRID} (in hex, in Internet-standard bit-
 order)
 The first three octets are derived from the IANA's Organizational
 Unique Identifier (OUI).  The next two octets (00-01) indicate the
 address block assigned to the VRRP for IPv4 protocol. {VRID} is the
 VRRP Virtual Router Identifier.  This mapping provides for up to 255
 IPv4 VRRP routers on a network.
 IPv6 case: 00-00-5E-00-02-{VRID} (in hex, in Internet-standard bit-
 order)
 The first three octets are derived from the IANA's OUI.  The next two
 octets (00-02) indicate the address block assigned to the VRRP for
 IPv6 protocol. {VRID} is the VRRP Virtual Router Identifier.  This
 mapping provides for up to 255 IPv6 VRRP routers on a network.

7.4. IPv6 Interface Identifiers

 IPv6 routers running VRRP MUST create their Interface Identifiers in
 the normal manner (e.g., "Transmission of IPv6 Packets over Ethernet
 Networks" [RFC2464]).  They MUST NOT use the virtual router MAC
 address to create the Modified Extended Unique Identifier (EUI)-64
 identifiers.
 This VRRP specification describes how to advertise and resolve the
 VRRP router's IPv6 link-local address and other associated IPv6
 addresses into the virtual router MAC address.

Nadas Standards Track [Page 28] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

8. Operational Issues

8.1. IPv4

8.1.1. 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 IPv4 source address of an ICMP redirect should be the address
 that 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.

8.1.2. Host ARP Requests

 When a host sends an ARP request for one of the virtual router IPv4
 addresses, the Virtual Router Master MUST respond to the ARP request
 with an ARP response that indicates the virtual MAC address for the
 virtual router.  Note that the source address of the Ethernet frame
 of this ARP response is the physical MAC address of the physical
 router.  The Virtual Router Master MUST NOT respond with its physical
 MAC address in the ARP response.  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 using its physical MAC address for the IPv4 address it owns;
 it should only send ARP messages that include virtual MAC addresses.
 This may entail the following:
 o  When configuring an interface, Virtual Router Master routers
    should broadcast a gratuitous ARP request containing the virtual
    router MAC address for each IPv4 address on that interface.
 o  At system boot, when initializing interfaces for VRRP operation,
    delay gratuitous ARP requests and ARP responses until both the
    IPv4 address and the virtual router MAC address are configured.

Nadas Standards Track [Page 29] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

 o  When, for example, ssh access to a particular VRRP router is
    required, an IP address known to belong to that router must be
    used.

8.1.3. 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.

8.2. IPv6

8.2.1. ICMPv6 Redirects

 ICMPv6 redirects may be used normally when VRRP is running between a
 group of routers [RFC4443].  This allows VRRP to be used in
 environments where the topology is not symmetric (e.g., the VRRP
 routers do not connect to the same destinations).
 The IPv6 source address of an ICMPv6 redirect should be the address
 that 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.  A method to deduce the virtual router used is to examine
 the destination MAC address in the packet that triggered the
 redirect.

8.2.2. ND Neighbor Solicitation

 When a host sends an ND Neighbor Solicitation message for the virtual
 router IPv6 address, the Virtual Router Master MUST respond to the ND
 Neighbor Solicitation message with the virtual MAC address for the
 virtual router.  The Virtual Router Master 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 Virtual Router Master sends an ND Neighbor Solicitation
 message for a host's IPv6 address, the Virtual Router Master MUST
 include the virtual MAC address for the virtual router if it sends a
 source link-layer address option in the neighbor solicitation
 message.  It MUST NOT use its physical MAC address in the source
 link-layer address option.
 When a VRRP router restarts or boots, it SHOULD NOT send any ND
 messages with its physical MAC address for the IPv6 address it owns;
 it should only send ND messages that include virtual MAC addresses.

Nadas Standards Track [Page 30] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

 This may entail the following:
 o  When configuring an interface, Virtual Router Master routers
    should send an unsolicited ND Neighbor Advertisement message
    containing the virtual router MAC address for the IPv6 address on
    that interface.
 o  At system boot, when initializing interfaces for VRRP operation,
    all ND Router and Neighbor Advertisements and Solicitation
    messages must be delayed until both the IPv6 address and the
    virtual router MAC address are configured.
 Note that on a restarting Master router where the VRRP protected
 address is the interface address, (that is, priority 255) duplicate
 address detection (DAD) may fail, as the Backup router may answer
 that it owns the address.  One solution is to not run DAD in this
 case.

8.2.3. Router Advertisements

 When a Backup VRRP router has become Master for a virtual router, it
 is responsible for sending Router Advertisements for the virtual
 router as specified in Section 6.4.3.  The Backup routers must be
 configured to send the same Router Advertisement options as the
 address owner.
 Router Advertisement options that advertise special services (e.g.,
 Home Agent Information Option) that are present in the address owner
 should not be sent by the address owner unless the Backup routers are
 prepared to assume these services in full and have a complete and
 synchronized database for this service.

8.3. IPvX

8.3.1. Potential Forwarding Loop

 If it is not the address owner, a VRRP router SHOULD NOT forward
 packets addressed to the IPvX address for which it becomes Master.
 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 IPvX TTL expires.
 One such mechanism for VRRP routers is to add/delete a reject host
 route for each adopted IPvX address when transitioning to/from MASTER
 state.

Nadas Standards Track [Page 31] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

8.3.2. Recommendations Regarding Setting Priority Values

 A priority value of 255 designates a particular router as the "IPvX
 address owner".  Care must be taken not to configure more than one
 router on the link in this way for a single VRID.
 Routers with priority 255 will, as soon as they start up, preempt all
 lower-priority routers.  No more than one router on the link is to be
 configured with priority 255, especially if preemption is set.  If no
 router has this priority, and preemption is disabled, then no
 preemption will occur.
 When there are multiple Backup routers, their priority values should
 be uniformly distributed.  For example, if one Backup router has the
 default priority of 100 and another Backup Router is added, a
 priority of 50 would be a better choice for it than 99 or 100, in
 order to facilitate faster convergence.

8.4. VRRPv3 and VRRPv2 Interoperation

8.4.1. Assumptions

 1. VRRPv2 and VRRPv3 interoperation is optional.
 2. Mixing VRRPv2 and VRRPv3 should only be done when transitioning
    from VRRPv2 to VRRPv3.  Mixing the two versions should not be
    considered a permanent solution.

8.4.2. VRRPv3 Support of VRRPv2

 As mentioned above, this support is intended for upgrade scenarios
 and is NOT recommended for permanent deployments.
 An implementation MAY implement a configuration flag that tells it to
 listen for and send both VRRPv2 and VRRPv3 advertisements.
 When a virtual router is configured this way and is the Master, it
 MUST send both types at the configured rate, even if sub-second.
 When a virtual router is configured this way and is the Backup, it
 should time out based on the rate advertised by the Master; in the
 case of a VRRPv2 Master, this means it must translate the timeout
 value it receives (in seconds) into centiseconds.  Also, a Backup
 should ignore VRRPv2 advertisements from the current Master if it is
 also receiving VRRPv3 packets from it.  It MAY report when a VRRPv3
 Master is *not* sending VRRPv2 packets: that suggests they don't
 agree on whether they're supporting VRRPv2 routers.

Nadas Standards Track [Page 32] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

8.4.3. VRRPv3 Support of VRRPv2 Considerations

8.4.3.1. Slow, High-Priority Masters

 See also Section 5.2.7, "Maximum Advertisement Interval
 (Max Adver Int)".
 The VRRPv2 Master router interacting with a sub-second VRRPv3 Backup
 router is the most important example of this.
 A VRRPv2 implementation should not be given a higher priority than a
 VRRPv2/VRRPv3 implementation it is interacting with if the VRRPv2/
 VRRPv3 rate is sub-second.

8.4.3.2. Overwhelming VRRPv2 Backups

 It seems possible that a VRRPv3 Master router sending at centisecond
 rates could potentially overwhelm a VRRPv2 Backup router with
 potentially unclear results.
 In this upgrade case, a deployment should initially run the VRRPv3
 Master routers with lower frequencies (e.g., 100 centiseconds) until
 the VRRPv2 routers are upgraded.  Then, once the deployment has
 convinced itself that VRRPv3 is working properly, the VRRPv2 support
 may be unconfigured and then the desired sub-second rates configured.

9. Security Considerations

 VRRP for IPvX does not currently include any type of authentication.
 Earlier versions of the VRRP (for IPv4) specification included
 several types of authentication ranging from none to strong.
 Operational experience and further analysis determined that these did
 not provide sufficient security to overcome the vulnerability of
 misconfigured secrets, causing multiple Masters to be elected.  Due
 to the nature of the VRRP protocol, even if VRRP messages are
 cryptographically protected, it does not prevent hostile nodes from
 behaving as if they are a VRRP Master, creating multiple Masters.
 Authentication of VRRP messages could have prevented a hostile node
 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 node could not disrupt VRRP, it can disrupt ARP and create
 the same effect as having all routers go into Backup.

Nadas Standards Track [Page 33] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

 Some L2 switches provide the capability to filter out, for example,
 ARP and/or ND messages from end-hosts on a switch-port basis.  This
 mechanism could also filter VRRP messages from switch ports
 associated with end-hosts and can be considered for deployments with
 untrusted hosts.
 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 router's MAC address;
 sending packets with the router's MAC address as the source MAC
 address 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 ND replies; answer ND requests; 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.
 In the context of IPv6 operation, if SEcure Neighbor Discovery (SEND)
 is deployed, VRRP is compatible with the "trust anchor" and "trust
 anchor or cga" modes of SEND [RFC3971].  The SEND configuration needs
 to give the Master and Backup routers the same prefix delegation in
 the certificates so that Master and Backup routers advertise the same
 set of subnet prefixes.  However, the Master and Backup routers
 should have their own key pairs to avoid private key sharing.

10. Contributors and Acknowledgments

 The editor would like to thank V. Ullanatt for his review of an early
 version.  This document consists of very little new material (there
 is some new text in Appendix A) and was created by merging and
 "xml-izing" [VRRP-IPv6] and [RFC3768], and then adding in the changes
 discussed recently on the Virtual Router Redundancy Protocol working
 group's mailing list.  R. Hinden is the author and J. Cruz the editor
 of the former.  The contributors for the latter appear below.

Nadas Standards Track [Page 34] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

 The IPv6 text in this specification is based on [RFC2338].  The
 authors of RFC2338 are S. Knight, D. Weaver, D. Whipple, R. Hinden,
 D. Mitzel, P. Hunt, P. Higginson, M. Shand, and A. Lindem.
 The author of [VRRP-IPv6] would also like to thank Erik Nordmark,
 Thomas Narten, Steve Deering, Radia Perlman, Danny Mitzel, Mukesh
 Gupta, Don Provan, Mark Hollinger, John Cruz, and Melissa Johnson for
 their helpful suggestions.
 The IPv4 text in this specification is based on [RFC3768].  The
 authors of that specification would like to thank Glen Zorn, 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.

11. IANA Considerations

 IANA has assigned an IPv6 link-local scope multicast address for VRRP
 for IPv6.  The IPv6 multicast address is as follows:
    FF02:0:0:0:0:0:0:12
 The values assigned address should be entered into Section 5.1.2.2.
 The IANA has reserved a block of IANA Ethernet unicast addresses for
 VRRP for IPv6 in the range
    00-00-5E-00-02-00 to 00-00-5E-00-02-FF (in hex)
 Similar assignments are documented at:
    http://www.iana.org

12. References

12.1. Normative References

 [ISO.10038.1993]  International Organization for Standardization,
                   "Information technology - Telecommunications and
                   information exchange between systems - Local area
                   networks - Media access control (MAC) bridges", ISO
                   Standard 10038, 1993.
 [RFC2119]         Bradner, S., "Key words for use in RFCs to Indicate
                   Requirement Levels", BCP 14, RFC 2119, March 1997.

Nadas Standards Track [Page 35] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

 [RFC2460]         Deering, S. and R. Hinden, "Internet Protocol,
                   Version 6 (IPv6) Specification", RFC 2460,
                   December 1998.
 [RFC3768]         Hinden, R., "Virtual Router Redundancy Protocol
                   (VRRP)", RFC 3768, April 2004.
 [RFC4291]         Hinden, R. and S. Deering, "IP Version 6 Addressing
                   Architecture", RFC 4291, February 2006.
 [RFC4443]         Conta, A., Deering, S., and M. Gupta, Ed.,
                   "Internet Control Message Protocol (ICMPv6) for the
                   Internet Protocol Version 6 (IPv6) Specification",
                   RFC 4443, March 2006.
 [RFC4861]         Narten, T., Nordmark, E., Simpson, W., and H.
                   Soliman, "Neighbor Discovery for IP version 6
                   (IPv6)", RFC 4861, September 2007.

12.2. Informative References

 [VRRP-IPv6]       Hinden, R. and J. Cruz, "Virtual Router Redundancy
                   Protocol for IPv6", Work in Progress, March 2007.
 [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.
 [RFC1071]         Braden, R., Borman, D., Partridge, C., and W.
                   Plummer, "Computing the Internet checksum", RFC
                   1071, September 1988.
 [RFC1256]         Deering, S., Ed., "ICMP Router Discovery Messages",
                   RFC 1256, September 1991.
 [RFC1469]         Pusateri, T., "IP Multicast over Token-Ring Local
                   Area Networks", RFC 1469, June 1993.
 [RFC2131]         Droms, R., "Dynamic Host Configuration Protocol",
                   RFC 2131, March 1997.
 [RFC2281]         Li, T., Cole, B., Morton, P., and D. Li, "Cisco Hot
                   Standby Router Protocol (HSRP)", RFC 2281, March
                   1998.

Nadas Standards Track [Page 36] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

 [RFC2328]         Moy, J., "OSPF Version 2", STD 54, RFC 2328, April
                   1998.
 [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.
 [RFC2453]         Malkin, G., "RIP Version 2", STD 56, RFC 2453,
                   November 1998.
 [RFC2464]         Crawford, M., "Transmission of IPv6 Packets over
                   Ethernet Networks", RFC 2464, December 1998.
 [RFC3971]         Arkko, J., Ed., Kempf, J., Zill, B., and P.
                   Nikander, "SEcure Neighbor Discovery (SEND)", RFC
                   3971, March 2005.
 [TKARCH]          IBM Incorporated, "IBM Token-Ring Network,
                   Architecture Specification, Publication
                   SC30-3374-02, Third Edition", September 1989.

Nadas Standards Track [Page 37] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

Appendix A. Operation over FDDI, Token Ring, and ATM LANE

A.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.

A.2. Operation over Token Ring

 Token Ring has several characteristics that make running VRRP
 difficult.  These include the following:
 o  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.
 o  No general multicast mechanism is 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.
 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-IPvX protocols using the same
 functional address.  Since the Novell IPX [IPX] protocol uses the

Nadas Standards Track [Page 38] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

 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-
 IPvX 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 [ISO.10038.1993].
 The functional-address mode of operation MUST be implemented by
 routers supporting VRRP on Token Ring.
 Additionally, routers MAY support the 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.  ND
 request/reply packets contain the virtual MAC address as the source
 MAC address.  The reason for this is that some Token-Ring driver

Nadas Standards Track [Page 39] RFC 5798 VRRPv3 for IPv4 and IPv6 March 2010

 implementations keep a cache of MAC address/source-routing
 information independent of the ND cache.
 Hence, these implementations have 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 ND cache and do not listen to
 gratuitous NDs, these hosts will not update their ND source-route
 information correctly when a switchover occurs.  The only possible
 solution is to put all routers with the same VRID on the same source-
 route-bridge segment and use techniques to prevent that bridge
 segment from being a single point of failure.  These techniques are
 beyond the scope of 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].

A.3. Operation over ATM LANE

 Operation of VRRP over ATM LANE on routers with ATM LANE interfaces
 and/or routers behind proxy LAN Emulation Clients (LECs) are beyond
 the scope of this document.

Author's Address

 Stephen Nadas (editor)
 Ericsson
 900 Chelmsford St., T3 4th Floor
 Lowell, MA  01851
 USA
 Phone: +1 978 275 7448
 EMail: stephen.nadas@ericsson.com

Nadas Standards Track [Page 40]

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