Virtual Router Redundancy Protocol (VRRP) specifies an election protocol that dynamically assigns responsibility for a virtual router to one of the VRRP routers on a LAN. The VRRP router controlling the IP address(es) associated with a virtual router is called the Master, and forwards packets sent to these IP addresses. The election process provides dynamic fail over in the forwarding responsibility should the Master become unavailable. This allows any of the virtual router IP addresses on the LAN to be used as the default first hop router by end-hosts. The advantage of using VRRP is a higher availability default path without requiring configuration of dynamic routing or router discovery protocols on every end-host.  VRRP packets are sent encapsulated in IP packets.

Using VRRP, a virtual IP addresscan be specified manually or with Dynamic Host Configuration Protocol (DHCP) as a default. A virtual IP address is shared among the routers, with one designated as the master router and the others as backups. In case, the master fails, the virtual IP address is mapped to a backup router's IP address. (This backup becomes the master router.) VRRP can also be used for load balancing. VRRP is part of both IPv4 and IPv6.
Protocol Structure - VRRP Virtual Router Redundancy Protocol

4	8	16	24	32bit
Version	Type	Virtual Rtr ID	Priority	Count IP Addrs
Auth Type		Advet Int	Checksum
IP Address 1
...
IP Address n
Authentication Data 1
Authentication Data 2

• Version-- The version field specifies the VRRP protocol version of this packet. This version is version 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. 
• Virtual Rtr ID -- The Virtual Router Identifier (VRID) field identifies the virtual router this packet is reporting status for.
• Priority -- Specifies the sending VRRP router's priority for the virtual router. VRRP routers backing up a virtual router MUST use priority values between 1-254 (decimal).
• Count IP Addresses --The number of IP addresses contained in this VRRP advertisement.
• Auth Type-- Identifies the authentication method being utilized.
• Advertisement Interval -- Indicates the time interval (in seconds) between advertisements.
• Checksum - used to detect data corruption in the VRRP message.
• 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.
• 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.

VRRP is defined by IETF (http://www.ietf.org ) RFC2338.

http://www.javvin.com/protocol/rfc2338.pdf : VRRP Specification