Virtual Router Redundancy Protocol (VRRP)
The Virtual Router Redundancy Protocol (VRRP) makes it possible to have one or more redundant routers as default gateway. With VRRP multiple routers appear as a single virtual router. VRRP is an Internet standard described in RFC 3768.
VRRP concepts:
VRRP Router
A router running the Virtual Router Redundancy Protocol. It may participate in 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 ID and a set of associated IP address(es) across a common LAN. A VRRP Router may backup one or more virtual routers.
Virtual Router ID
The ID of a Virtual Router (VRID).
Virtual Router IP
The IP address of a Virtual Router
Virtual MAC address
The MAC address for a Virtual Router. There are a number of reserved MAC addresses for the Virtual Router functionality in format: 00-00-5E-00-01-{VRID} (in hex in Internet standard bitorder). {VRID} is the VRRP Virtual Router Identifier. This mapping provides for up to 255 VRRP routers on a network. For details see RFC 3768.
Master
The VRRP router that is assuming the responsibility of forwarding packets sent to the IP address(es) associated with the virtual router, and answering ARP requests for these IP addresses.
Backup
Above is a picture with a network that implements one virtual router on the outgoing side (VRID 1) and another virtual router on the incoming side (VRID2). With this setup it is possible to have resilient routing both into and out of the domain without having to run a dynamic routing protocol.