rfc2338.txt

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   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.







Knight, et. al.             Standards Track                     [Page 7]

RFC 2338                          VRRP                        April 1998


4.  Sample Configurations

4.1  Sample Configuration 1

   The following figure shows a simple network with two VRRP routers
   implementing one virtual router.  Note that this example is provided
   to help understand the protocol, but is not expected to occur in
   actual practice.

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

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

   The above configuration shows a very simple VRRP scenario.  In this
   configuration, the end-hosts install a default route to the IP
   address of virtual router #1 (IP A) and both routers run VRRP.  The
   router on the left becomes the Master for virtual router #1 (VRID=1)
   and the router on the right is the Backup for virtual router #1.  If
   the router on the left should fail, the other router will take over
   virtual router #1 and its IP addresses, and provide uninterrupted
   service for the hosts.

   Note that in this example, IP B is not backed up by the router on the
   left.  IP B is only used by the router on the right as its interface
   address.  In order to backup IP B, a second virtual router would have
   to be configured.  This is shown in the next section.



Knight, et. al.             Standards Track                     [Page 8]

RFC 2338                          VRRP                        April 1998


4.2  Sample Configuration 2

   The following figure shows a configuration with two virtual routers
   with the hosts spitting their traffic between them.  This example is
   expected to be very common in actual practice.

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

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

   In the 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.

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
   the IPv4 multicast address assigned to VRRP.





Knight, et. al.             Standards Track                     [Page 9]

RFC 2338                          VRRP                        April 1998


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

       224.0.0.18

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

5.2.3  TTL

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





Knight, et. al.             Standards Track                    [Page 10]

RFC 2338                          VRRP                        April 1998


5.2.4  Protocol

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

5.3 VRRP Field Descriptions

5.3.1  Version

   The version field specifies the VRRP protocol version of this packet.
   This document defines version 2.

5.3.2  Type

   The type field specifies the type of this VRRP packet.  The only
   packet type defined in this version of the protocol is:

       1      ADVERTISEMENT

   A packet with unknown type MUST be discarded.

5.3.3  Virtual Rtr ID (VRID)

   The Virtual Router Identifier (VRID) field identifies the virtual
   router this packet is reporting status for.

5.3.4  Priority

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

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

   VRRP routers backing up a virtual router MUST use priority values
   between 1-254 (decimal).  The default priority value for VRRP routers
   backing up a virtual router is 100 (decimal).

   The priority value zero (0) has special meaning indicating that the
   current Master has stopped participating in VRRP.  This is used to
   trigger Backup routers to quickly transition to Master without having
   to wait for the current Master to timeout.

5.3.5  Count IP Addrs

   The number of IP addresses contained in this VRRP advertisement.




Knight, et. al.             Standards Track                    [Page 11]

RFC 2338                          VRRP                        April 1998


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 unsigned integer.
   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.

   Note that there are security implications to using Simple Text
   password authentication, and one should see the Security
   Consideration section of this document.

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.



Knight, et. al.             Standards Track                    [Page 12]

RFC 2338                          VRRP                        April 1998


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 received with an authentication string that does not
   match the locally configured authentication string MUST be discarded.
   The authentication string is unique on a per interface basis.

   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.




Knight, et. al.             Standards Track                    [Page 13]

RFC 2338                          VRRP                        April 1998


6.1.2 Parameters per Virtual Router

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

   Priority                Priority value to be used by this VRRP
                           router in Master election for this virtual
                           router.  The value of 255 (decimal) is
                           reserved for the router that owns the IP
                           addresses associated with the virtual
                           router.  The value of 0 (zero) is reserved
                           for Master router to indicate it is
                           releasing responsibility for the virtual
                           router.  The range 1-254 (decimal) is
                           available for VRRP routers backing up the
                           virtual router.  The default value is 100
                           (decimal).

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

   Advertisement_Interval  Time interval between ADVERTISEMENTS