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Versions: 00 01 02 03 04 05 RFC 2338

INTERNET-DRAFT                                                 S. Knight
October 23, 1997                                               D. Weaver
                                             Ascend Communications, Inc.
                                                              D. Whipple
                                                         Microsoft, Inc.
                                                               R. Hinden
                                                               D. Mitzel
                                                                 P. Hunt
                                                  Ipsilon Networks, Inc.
                                                            P. Higginson
                                                                M. Shand
                                                 Digital Equipment Corp.



                   Virtual Router Redundancy Protocol

                     <draft-ietf-vrrp-spec-03.txt>



Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   To learn the current status of any Internet-Draft, please check the
   "1id-abstracts.txt" listing contained in the Internet- Drafts Shadow
   Directories on ds.internic.net (US East Coast), nic.nordu.net
   (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
   Rim).

   This internet draft expires on April 23, 1998.

Abstract

   This memo defines the Virtual Router Redundancy Protocol (VRRP).
   VRRP specifies an election protocol that dynamically allows a set of
   routers running VRRP to backup each other on a LAN.  The VRRP router
   controlling one or more IP addresses is called the Master router, and
   forwards packets sent to these IP addresses.  The election process



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   provides dynamic fail over in the forwarding responsibility should
   the Master become unavailable.  This allows any of the VRRP routers
   IP addresses on the LAN to be used as the default first hop router by
   end-hosts.  The advantage gained from using the 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...............................................3
   2.  Required Features..........................................5
   3.  VRRP Overview..............................................6
   4.  Sample Configurations......................................8
   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....................................14
      6.1 Parameters.............................................14
      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...................................18
      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.............................................21
   9.  Operation over FDDI and Token Ring........................21
   10. Security Considerations...................................22
      10.1 No Authentication.....................................22
      10.2 Simple Text Password..................................22
      10.3 IP Authentication Header..............................22
   11. Acknowledgments...........................................23
   12. References................................................24
   13. Authors' Addresses........................................24
   14. Changes from Previous Drafts..............................26











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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
   of large numbers of hosts.  This can result in a significant delay in
   the detection of a lost (i.e., dead) neighbor, which 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 allows a set of routers to backup each other.  The VRRP
   router controlling one or more IP addresses is called the Master
   router, 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 IP
   addresses on a virtual router can then be used as the default first
   hop router by end-hosts.  The advantage gained from using the 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 a Cisco Systems, Inc. proprietary
   protocol named Hot Standby Router Protocol (HSRP) [HSRP] and to a
   Digital Equipment Corporation, Inc. proprietary protocol named IP
   Standby Protocol.

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



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   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC 2119].


1.1  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 Master router are presented.  Finally, operational issues
   related to MAC address mapping, handling of ARP requests, generation
   of ICMP redirect messages, and security issues are addressed.

   This protocol is intended for use with IPv4 routers only.  A separate
   specification will be produced if it is decided that similar
   functionality is desirable in an IPv6 environment.


1.2  Definitions

   Virtual Router       One of a set of routers running VRRP on a LAN.

   IP Address Owner     The virtual router than has the IP address(es)
                        as real interface address(es).  This is the
                        router that, when up, will respond to packets
                        addressed to one of these IP addresses for ICMP
                        pings, TCP connections, etc.

   Primary IP Address   An IP address selected from the set of real
                        interface addresses.  One possible selection
                        algorithm is to always select the first address.
                        VRRP advertisements are always sent using the
                        primary IP address as the source of the IP
                        packet.

   Master Router        The virtual router that is assuming the
                        responsibility of forwarding packets sent to the
                        IP addresses associated with a virtual router
                        and answering ARP requests for these IP
                        addresses.  The Master Router may or may not be
                        the owner.  Note that if the IP address owner is
                        available, then it will always be the master
                        router.

   Backup Router        The set of virtual routers available to assume
                        forwarding responsibility for a virtual router
                        should the current master router fail.




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2.0 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 Master router and
   the additional functionality described below, the protocol should
   strive to:

    - Minimize the duration of black holes.
    - Minimize the steady state bandwidth overhead and processing
      complexity.
    - Function over a wide variety of multiaccess LAN technologies
      capable of supporting IP traffic.
    - Provide for election of multiple virtual routers on a network for
      load balancing or in 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.

   Some environments may find it beneficial to avoid the state
   transition triggered when a router becomes available that is more



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   preferential than the current Master.  It may be useful to support an
   override of the immediate convergence to the preferred path.


2.4 Extensible Security

   The virtual router functionality is applicable to a wide range of
   internetworking environments that may employ different security
   policies.  The protocol should require minimal configuration and
   overhead in the insecure operation, provide for strong authentication
   when increased security is required, and allow integration of new
   security mechanisms without breaking backwards compatible operation.


2.5 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.


3.0 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 messages sent by
   the Master router to enable bridge learning in an extended LAN.

   A virtual router is identified by its virtual router identifier.  A
   VRRP router has a set of addresses that it owns and one or more other
   virtual routers it is responsible for backing up.  On an interface
   running VRRP, each VRRP router must be configured with a virtual
   router identifier for the addresses it owns, and the other virtual



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   router identifiers and associated IP addresses that it is responsible
   for backing up.  In addition, each VRRP router is assigned a priority
   to indicate it's preference in Master election for each virtual
   router.  Multiple virtual routers can be elected on a network and a
   single router can backup one or more virtual routers.

   To minimize network traffic, only the Master router sends periodic
   Advertisement messages.  A Backup router will not attempt to pre-empt
   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 pre-emption attempts.  The
   only exception to this is that the owner will always become master
   when it is up.  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.

   VRRP defines three types of authentication providing simple
   deployment in insecure environments, added protection against
   misconfiguration, and strong sender authentication in security
   conscious environments.  Analysis of the protection provided and
   vulnerability of each mechanism is deferred to Section 10.0 Security
   Considerations.  In addition new authentication types and data can be
   defined in the future without affecting the format of the fixed
   portion of the protocol packet, thus preserving backward compatible
   operation.

   The VRRP protocol design provides rapid transition from Backup to
   Master to minimize service interruption, and incorporates
   optimizations that reduce protocol complexity while guaranteeing
   controlled Master transition for typical operational scenarios.  The
   optimizations result in an election protocol with minimal runtime
   state requirements, minimal active protocol states, and a single
   message type and sender.  The typical operational scenarios are
   defined to be two redundant routers and/or distinct path preferences
   among each router.  A side effect when these assumptions are violated
   (i.e., more than two redundant paths all with equal preference) is
   that duplicate packets may be forwarded for a brief period during
   Master election.  However, the typical scenario assumptions are
   likely to cover the vast majority of deployments, loss of the Master
   router is infrequent, and the expected duration in Master election
   convergence is quite small ( << 1 second ).  Thus the VRRP
   optimizations represent significant simplifications in the protocol
   design while incurring an insignificant probability of brief network
   degradation.






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4.  Sample Configurations

4.1  Sample Configuration 1

   The following figure shows a simple network with two virtual routers.

                       VRID=1       VRID=2
                      +-----+      +-----+
                      | MR1 |      | MR2 |
                      |  &  |      |  &  |
                      | BR2 |      | BR1 |
                      +-----+      +-----+
         IP A ---------->*            *<---------- IP B
                         |            |
                         |            |
                         |            |
       ------------------+------------+-----+--------+--------+--------+--
                                            ^        ^        ^        ^
                                            |        |        |        |
                                          (IP A)   (IP A)   (IP A)   (IP A)
                                            |        |        |        |
                                         +--+--+  +--+--+  +--+--+  +--+--+
                                         |  H1 |  |  H2 |  |  H3 |  |  H4 |
                                         +-----+  +-----+  +--+--+  +--+--+

      Legend:
               ---+---+---+--  =  802 network, Ethernet or FDDI
                            H  =  Host computer
                           MR  =  Master Router
                           BR  =  Backup Router
                            *  =  IP Address
                         (IP)  =  default router for hosts

   The above configuration shows a simple VRRP scenario.  In this
   configuration, the end-hosts install a default route to the IP
   address of one of the virtual routers (IP A) and the routers run
   VRRP.  The router on the left (VRID=1) becomes the Master router for
   the IP addresses it owns (IP A) and the router on the right (VRID=2)
   becomes the Master router for the IP addresses it owns (IP B).  Each
   router also backs up the other router.  If the router on the left
   (VRID=1) should fail, the other router will take over its IP
   addresses and provide uninterrupted service for the hosts.









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4.2  Sample Configuration 2

   The following figure shows a configuration with two virtual routers
   with the hosts slitting their traffic between them.

                       VRID=1       VRID=2
                      +-----+      +-----+
                      | MR1 |      | MR2 |
                      |  &  |      |  &  |
                      | BR2 |      | BR1 |
                      +-----+      +-----+
         IP A ---------->*            *<---------- IP B
                         |            |
                         |            |
                         |            |
       ------------------+------------+-----+--------+--------+--------+--
                                            ^        ^        ^        ^
                                            |        |        |        |
                                          (IP A)   (IP A)   (IP B)   (IP B)
                                            |        |        |        |
                                         +--+--+  +--+--+  +--+--+  +--+--+
                                         |  H1 |  |  H2 |  |  H3 |  |  H4 |
                                         +-----+  +-----+  +--+--+  +--+--+

      Legend:
               ---+---+---+--  =  802 network, Ethernet or FDDI
                            H  =  Host computer
                           MR  =  Master Router
                           BR  =  Backup Router
                            *  =  IP Address
                         (IP)  =  default router for hosts

   In the above configuration, half of the hosts install a default route
   to virtual router 1's IP address (IP A), and the other half of the
   hosts install a default route to virtual router 2's IP address (IP
   B).  This has the effect of load balancing the outgoing traffic,
   while also providing full redundancy.














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5.0  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
   an IPv4 multicast address assigned to VRRP.

5.1  VRRP Packet Format

   This section defines the format of the VRRP packet and the relevant
   fields in the IP header.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Version| Type  | Virtual Rtr ID|   Priority    | Count IP Addrs|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Auth Type   |   Adver Int   |          Checksum             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         IP Address (1)                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            .                                  |
      |                            .                                  |
      |                            .                                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         IP Address (n)                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Authentication Data (1)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Authentication Data (2)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.2  IP Field Descriptions

5.2.1  Source Address

   The primary IP address of the interface the packet is being sent
   from.

5.2.2  Destination Address

   The VRRP IP multicast address assigned by the IANA.  It is defined to
   be:

       224.0.0.(TBD IANA assignment)




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

5.2.3  TTL

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

5.2.4  Protocol

   The VRRP IP protocol number assigned by the IANA.  It is defined to
   be (TBD).


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.


5.3.4  Priority

   The priority field specifies the router's priority for the virtual
   router.  Higher values equal higher priority.  This field is an 8 bit
   unsigned field.

   The priority value for the router that owns the IP address(es)
   associated with the virtual router MUST be 255 (decimal).

   VRRP routers backing up another virtual router MAY use priority



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   values between 1-254 (decimal).  The default priority value for
   routers backing up another 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 per interface
   basis.  The authentication type field is an 8 bit number.  A packet
   with unknown authentication type or that does not match the locally
   configured authentication method MUST be discarded.

   The authentication methods currently defined are:

      0 - No Authentication
      1 - Simple Text Password
      2 - IP Authentication Header

5.3.6.1 No Authentication

   The use of this authentication type means that VRRP protocol
   exchanges are not authenticated.  The contents of the Authentication
   Data field should be set to zero on transmission and ignored on
   reception.

5.3.6.2 Simple Text Password

   The use of this authentication type means that VRRP protocol
   exchanges are authenticated by a clear text password.  The contents
   of the Authentication Data field should be set to the locally
   configured password on transmission.  There is no default password.
   The receiver MUST check that the Authentication Data in the packet
   matches its configured authentication string.  Packets that do not
   match MUST be discarded.







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5.3.6.3 IP Authentication Header

   The use of this authentication type means the VRRP protocol exchanges
   are authenticated using the mechanisms defined by the IP
   Authentication Header [AUTH] using "The Use of HMAC-MD5-96 within ESP
   and AH" [HMAC].  Keys may be either configured manually or via a key
   distribution protocol.

   If a packet is received that does not pass the authentication check
   due to a missing authentication header or incorrect message digest,
   then the packet MUST be discarded.  The contents of the
   Authentication Data field should be set to zero on transmission and
   ignored on reception.

5.3.7 Advertisement Interval (Adver Int)

   The Advertisement interval indicates the time interval (in seconds)
   between ADVERTISEMENTS.  The default is 1 second.  This field is used
   for troubleshooting misconfigured routers.

5.3.8 Checksum

   The checksum field is used to detect data corruption in the VRRP
   message.

   The checksum is the 16-bit one's complement of the one's complement
   sum of the entire VRRP message starting with the version field.  For
   computing the checksum, the checksum field is set to zero.


5.3.9  IP Address(es)

   One or more IP addresses that are associated with the virtual router.
   The number of addresses included is specified in the "Count IP Addrs"
   field.  These fields are used for troubleshooting misconfigured
   routers.


5.3.10  Authentication Data

   The authentication string is currently only utilized for simple text
   authentication, similar to the simple text authentication found in
   the Open Shortest Path First routing protocol [OSPF].  It is up to 8
   characters of plain text.  If the configured authentication string is
   shorter than 8 bytes, the remaining space MUST be zero-filled.  Any
   VRRP packet with an authentication string that does not match its
   configured authentication string SHOULD be discarded. The
   authentication string is unique on a per interface basis.



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   There is no default value for this field.


6.  Protocol State Machine

6.1 Parameters

6.1.1 Parameters per Interface


    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.


6.1.2 Parameters per Virtual Router


    Virtual Router Identifier.  Configured item in the range 1-255
                            (decimal).  There is no default.

    Priority                Priority value to be used 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 router to indicate it
                            has stopped running VRRP.  The range 1-254
                            (decimal) is available for VRRP routers
                            backing up the virtual router.  The default
                            value is 100 (decimal).

    IP_Addresses            One or more IP addresses associated with
                            this virtual router.  Configured item.  No
                            default.

    Advertisement_Interval  Time interval between ADVERTISEMENTS
                            (seconds).  Default is 1 second.

    Skew_Time               Time to skew Master_Down_Interval in
                            seconds.  Calculated as:

                               ( (256 - Priority) / 256 )






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    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 preempt and False to not
                            preempt.  Default is True.

                            Note: Exception is that the router that owns
                            the IP address(es) associated with the
                            virtual router always pre-empts independent
                            of the setting of this flag.


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.



6.3  State Transition Diagram

                          +---------------+
               +--------->|               |<-------------+
               |          |  Initialize   |              |
               |   +------|               |----------+   |
               |   |      +---------------+          |   |
               |   |                                 |   |
               |   V                                 V   |
       +---------------+                       +---------------+
       |               |---------------------->|               |
       |    Master     |                       |    Backup     |
       |               |<----------------------|               |
       +---------------+                       +---------------+


6.4  State Descriptions

   In the state descriptions below, the state names are identified by
   {state-name}, and the packets are identified by all upper case
   characters.



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6.4.1   Initialize

   {Initialize} is the state a virtual router takes when it is inactive
   with respect to the virtual router.  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 Send a gratuitous ARP request containing the virtual router MAC
         address for each IP address associated with the virtual router.
       o Set the Adver_Timer to Advertisement_Interval
       o Transition to the {Master} state

      else

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

      endif

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, an virtual router MUST do the following:

    - MUST NOT respond to ARP requests for the IP address(s) associated
      with this VRID.

    - MUST discard packets with a destination link layer MAC address
      equal to the virtual router MAC address for this VRID.

    - MUST NOT accept packets addressed to the IP address(es) associated
      with this VRID.

    - If a Shutdown event is received, then:

       o Cancel the Master_Down_Timer
       o Transition to the {Initialize} state

      endif







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    - If the Master_Down_Timer fires, then:

       o Send an ADVERTISEMENT
       o Send a gratuitous ARP request containing their virtual router
         MAC address for each IP address associated with the virtual
         router
       o Set the Adver_Timer to Advertisement_Interval
       o Transition to the {Master} state

      endif

    - If an ADVERTISEMENT is received, then:

         If the Priority in the ADVERTISEMENT is Zero, then:

          o Set the Master_Down_Timer to Skew_Time

         else:

            If Preempt_Mode is False, or If the Priority in the
            ADVERTISEMENT is greater than or equal to the local
            Priority, then:

             o Reset the Master_Down_Timer to Master_Down_Interval

            else:

             o Discard the ADVERTISEMENT

            endif
         endif
      endif


6.4.3   Master

   While in the {Master} state the router functions as the forwarding
   router for the IP address(es) associated with the virtual router.

   While in this state, a virtual router MUST do the following:

    - MUST respond to ARP requests for the IP address(es) associated
      with the VRID with the virtual router MAC address.

    - 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



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      for the virtual router if it is not the IP address owner.

    - MUST accept packets addressed to the IP address(es) if it is the
      IP address owner.

    - If a Shutdown event is received, then:

       o Cancel the Adver_Timer
       o Send an ADVERTISEMENT with Priority = 0
       o Transition to the {Initialize} state

      endif

    - If the Adver_Timer fires, then:

       o Send an ADVERTISEMENT
       o Reset the Adver_Timer to Advertisement_Interval

      endif

    - If an ADVERTISEMENT is received, then:

         If the Priority in the ADVERTISEMENT is Zero, then:

          o Send an ADVERTISEMENT
          o Reset the Adver_Timer to Advertisement_Interval

         else:

            If the Priority in the ADVERTISEMENT is greater than the
            local Priority,
            or
            If the Priority in the ADVERTISEMENT is equal to the local
            Priority and the primary IP Address of the sender is greater
            than the local primary IP Address, then:

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

            else:

             o Discard ADVERTISEMENT

            endif
         endif
      endif




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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 that the received packet length is greater than or
        equal to the VRRP header
      - MUST verify the VRRP checksum
      - MUST verify the VRRP version
      - MUST perform authentication specified by Auth Type

   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.

      - MUST verify that the VRID is valid on the receiving interface

   If the above checks fails, the receiver MUST discard the packet.

      - MAY verify that the IP address(es) associated with the VRID are
        valid

   If the above check fails, the receiver SHOULD log the event and MAY
   indicate via network management that an error occurred.  If the
   Priority does not equal 255 (decimal), the receiver MUST drop the
   packet.  If the Priority equals 255 (decimal) 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 an
   error occurred.


7.2 Transmitting Packets

   The following operations MUST be performed prior to transmitting a
   VRRP packet.

      - Fill in the VRRP packet fields with the appropriate virtual
        router configuration state
      - Compute the VRRP checksum
      - Set the source MAC address to Virtual Router MAC Address
      - Set the source IP address to interface primary IP address
      - Send the VRRP packet to the VRRP IP multicast group



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   Note: VRRP packets are transmitted with the virtual router MAC
   address as the source MAC address to ensure that learning bridges
   correctly determine the LAN segment the virtual router is attached
   to.


7.3 Virtual Router MAC Address

   The virtual router MAC address associated with a virtual router is an
   IEEE 802 MAC Address in the following format:

   00-00-5E-XX-XX-{VRID} (in hex in internet standard bit-order)

   The first three octets are derived from the IANA's OUI.  The next two
   octets (to be assigned by the IANA) indicate the address block
   assigned to the VRRP protocol.  {VRID} is the VRRP Router Identifier.
   This mapping provides for up to 255 VRRP routers on a network.


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.

   When acting as a Master for a VRID it is not the owner, the virtual
   router MUST send ICMP Redirects using the IP address associated with
   the VRID as the source of the ICMP Redirect.  This entails looking at
   the destination MAC address in the packet that is being redirected
   and selecting the appropriate IP address.

   It may be useful to disable Redirects for specific cases where is
   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 routers IP
   addresses, the Master router MUST respond to the ARP request with the
   virtual MAC address for the virtual router.  The virtual router MUST
   NOT respond with it's physical MAC address.  This allows the client
   to always use the same MAC address regardless of the current Master
   router.  The request MUST be handled as a standard ARP reply.

   When a virtual router restarts or boots, it SHOULD not send any ARP



draft-ietf-vrrp-spec-03.txt                                    [Page 20]


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   messages with it's physical MAC addresses for the IP addresses it
   owns.  They should only send ARP messages that include Virtual MAC
   addresses.  This may entail:

    - When configuring their interfaces, virtual routers should send a
      gratuitous ARP request containing their virtual MAC address for
      each IP address they own on that interface.

    - At system boot, when initializing  any of its IP addresses for
      which VRRP is configured, delay gratuitous ARP requests and ARP
      responses for that interface until both the IP address and the
      virtual MAC address are configured.


8.3 Proxy ARP

   If Proxy ARP is to be used on a router running VRRP, 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 routers.


9.  Operation over FDDI and Token Ring

9.1 Operation over FDDI

   FDDI interfaces strip 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 stripped off 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 stripping any ADVERTISEMENTS it did not originate.


9.2  Operation over Token Ring

   Token Ring has several characteristics which make running VRRP
   problematic.  This includes:

    - No general multicast mechanism.  Required use of "functional
      addresses" as a substitute, which may collide with other usage of



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      the same "functional addresses".
    - Token Ring interfaces may have a limited ability to receive on
      multiple MAC addresses.
    - In order to switch to a new master located on a different physical
      ring from the previous master when using source route bridges, a
      mechanism is required to update cached source route information.

   Due the these issues and the limited knowledge about the detailed
   operation of Token Ring by the authors, this version of VRRP does not
   work over Token Ring networks.  This may be remedied in new version
   of this document, or in a separate document.


10. Security Considerations

   VRRP is designed for a range of internetworking environments that may
   employ different security policies.  The protocol includes several
   authentication methods ranging from no authentication, simple clear
   text passwords, and strong authentication using IP Authentication
   with MD5 HMAC.  The details on each approach including possible
   attacks and recommended environments follows.

   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.


10.1 No Authentication

   The use of this authentication type means that VRRP protocol
   exchanges are not authenticated.  This type of authentication SHOULD
   only be used in environments were there is minimal security risk and
   little chance for configuration errors (e.g., two VRRP routers on a
   link).


10.2 Simple Text Password

   The use of this authentication type means that VRRP protocol
   exchanges are authenticated by a simple clear text password.

   This type of authentication is useful to protect against accidental
   misconfiguration of routers on a link.  It protects against routers
   inadvertently backing up another router.  A new router must first be
   configured with the correct password before it can run VRRP with
   another router.  This type of authentication does not protect against
   hostile attacks where the password can be learned by a node snooping



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   VRRP packets on the link.  The Simple Text Authentication combined
   with the TTL check makes it difficult for a VRRP packet to be sent
   from another link to disrupt VRRP operation.

   This type of authentication is RECOMMENDED when there is minimal risk
   of nodes on the link actively disrupting VRRP operation.


10.3 IP Authentication Header

   The use of this authentication type means the VRRP protocol exchanges
   are authenticated using the mechanisms defined by the IP
   Authentication Header [AUTH] using "The Use of HMAC-MD5-96 within ESP
   and AH", [HMAC].  This provides strong protection against
   configuration errors, replay attacks, and packet
   corruption/modification.

   This type of authentication is RECOMMENDED when there is limited
   control over the administration of nodes on the link.  While this
   type of authentication does protect the operation of VRRP, there are
   other types of attacks that may be employed on shared media links
   (e.g., generation of bogus ARP replies) which are independent from
   VRRP and are not protected.


11. Acknowledgments

   The authors would like to thank Glen Zorn, and Michael Lane, Clark
   Bremer, Hal Peterson, Tony Li, Barbara Denny, Joel Halpern, Steve
   Bellovin, and Acee Lindem for their comments and suggestions.





















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12.  References


   [AUTH]    Atkinson, R., "IP Authentication Header", RFC-1826, August
             1995.

   [DISC]    Deering, S., "ICMP Router Discovery Messages", RFC-1256,
             September 1991.

   [DHCP]    Droms, R., "Dynamic Host Configuration Protocol", RFC-1541,
             October 1993.

   [HMAC]    Madson, C., R. Glenn, "The Use of HMAC-MD5-96 within ESP
             and AH", Internet Draft, <draft-ietf-ipsec-auth-hmac-
             md5-96-00.txt> , July 1997.

   [HSRP]    Li, T., B. Cole, P. Morton, D. Li, "Hot Standby Router
             Protocol (HSRP)", Internet Draft, <draft-li-hsrp-00.txt>,
             June 1997.

   [OSPF]    Moy, J., "OSPF version 2", RFC-1583, July 1997.

   [RIP]     Hedrick, C., "Routing Information Protocol" , RFC-1058,
             June 1988.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", RFC-2119, BCP14, March 1997.


13. Author's Addresses

   Steven Knight                           Phone: +1 612 943-8990
   Ascend Communications                   EMail: Steven.Knight@ascend.com
   High Performance Network Division
   10250 Valley View Road, Suite 113
   Eden Prairie, MN USA 55344
   USA

   Douglas Weaver                          Phone: +1 612 943-8990
   Ascend Communications                   EMail: Doug.Weaver@ascend.com
   High Performance Network Division
   10250 Valley View Road, Suite 113
   Eden Prairie, MN USA 55344
   USA







draft-ietf-vrrp-spec-03.txt                                    [Page 24]


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   David Whipple                           Phone: +1 206 703-3876
   Microsoft Corporation                   EMail: dwhipple@microsoft.com
   One Microsoft Way
   Redmond, WA USA 98052-6399
   USA

   Robert Hinden                           Phone: +1 408 990-2004
   Ipsilon Networks, Inc.                  EMail: hinden@ipsilon.com
   232 Java Drive
   Sunnyvale, CA 94089
   USA

   Danny Mitzel                            Phone: +1 408 990-2037
   Ipsilon Networks, Inc.                  EMail: mitzel@ipsilon.com
   232 Java Drive
   Sunnyvale, CA 94089
   USA

   Peter Hunt                              Phone: +1 408 990-2093
   Ipsilon Networks, Inc.                  EMail: hunt@ipsilon.com
   232 Java Drive
   Sunnyvale, CA 94089
   USA

   P. Higginson                            Phone: +44 118 920 6293
   REO2-F/E9                               EMail: higginson@mail.dec.com
   Digital Equipment Corp.
   Digital Park
   Imperial Way
   Reading
   Berkshire
   RG2 0TE
   UK

   M. Shand                                Phone: +44 118 920 4424
   REO2-F/D9                               EMail: shand@mail.dec.com
   Digital Equipment Corp.
   Digital Park
   Imperial Way
   Reading
   Berkshire
   RG2 0TE
   UK








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14. Changes from Previous Drafts

   Changes from <draft-ietf-vrrp-spec-02.txt>

    - Updated text and references to point to "The Use of HMAC-MD5-96
      within ESP and AH" that is the correct reference for the use of
      IPSEC AH with MD5.


   Changes from <draft-ietf-vrrp-spec-01.txt>

   Major change to use real IP addresses instead of virtual IP
   addresses.  Changes include:

    - Updated version number to 2.
    - Modified packet header
    - New terminology (removed cluster, virtual IP address, etc., added
      VRID, associated IP address(es), etc.).
    - Special case of priority = 255 for router owning VRID and
      associated IP address(es).
    - Reworked examples.
    - Rewrote introductory and overview sections.
    - Added rules for redirects and ARP.
    - Added sending gratuitous ARP request when transitioning to Master.


   Changes from <draft-ietf-vrrp-spec-00.txt>

    - Added Preempt_Mode to allow user control over preemption
      independent of configured priorities.
    - Rewrote authentication section and expanded security
      considerations.
    - Expanded State Description section and removed State Table which
      become redundant and impossible to edit.
    - Changed authentication to be on a per interface basis (not per
      cluster).
    - Clarified text on disabling ICMP Redirects.
    - Added text on FDDI and Token Ring issues.
    - Added HSRP acknowledgment.
    - Rewrote Introduction, Required Features, and VRRP Overview
      sections.
    - Many small text clarifications.


   Changes from <draft-hinden-vrrp-00.txt>

    - Changed default behavior to stay with current master when
      priorities are equal.  This behavior can be changed by configuring



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      explicit priorities.
    - Changed Master state behavior to not send Advertisements when
      receiving Advertisement with lower priority.  Change reduces worst
      case election message overhead to "n", where "n" is number of
      configured equal priority VRRP routers.
    - Added Skew_Time parameter and changed receiving advertisement with
      zero priority behavior to cause resulting advertisement sent to be
      skewed by priority.
    - Changed sending behavior to send VRRP packets with VMAC as source
      MAC and added text describing why this is important for bridged
      environments.
    - Changed definition of VMAC to be in IANA assigned unicast MAC
      block.
    - Added Advertisement Interval to VRRP header.
    - Added text regarding ICMP Redirects, Proxy ARP, and network
      management issues.
    - Various small text clarifications.


































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