The Virtual Router Redundancy Protocol (VRRP) allows for transparent failover at the first hop IP router, by configuring a group of routers to share a virtual IP address. VRRP selects a master router in that group to handle all packets for the virtual IP address. The remaining routers are in standby and take over in the event that the master router fails.

VRRP Operation

A LAN client can determine which router should be the first hop to a particular remote destination by using a dynamic process or static configuration. Examples of dynamic router discovery are as follows:

 Proxy ARP: The client uses the Address Resolution Protocol (ARP) to get the destination it wants to reach, and a router will respond to the ARP request with its own MAC address.

 Routing protocol: The client listens to dynamic routing protocol updates (for example, from the Routing Information Protocol [RIP]) and forms its own routing table.

 ICMP Router Discovery Protocol (IRDP) client: The client runs an Internet Control Message Protocol (ICMP) router discovery client.

The disadvantage to dynamic discovery protocols is that they incur some configuration and processing overhead on the LAN client. Also, in the event of a router failure, the process of switching to another router can be slow.

An alternative to dynamic discovery protocols is to statically configure a default router on the client. Although this approach simplifies client configuration and processing, it creates a single point of failure. If the default gateway fails, the LAN client is limited to communicating only on the local IP network segment and is cut off from the rest of the network.

VRRP can solve the static configuration problem by enabling a group of routers (a VRRP group) to share a single virtual IP address. You can then configure the LAN clients with the virtual IP address as their default gateway.