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 19984.  Sample Configurations4.1  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 19984.2  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 19985.1  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 Descriptions5.2.1  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 19985.2.4  Protocol   The IP protocol number assigned by the IANA for VRRP is 112   (decimal).5.3 VRRP Field Descriptions5.3.1  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 19985.3.6  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 Header5.3.6.1 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 19985.3.7 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 Machine6.1 Parameters6.1.1 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 19986.1.2 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