Virtual Router Redundancy Protocol (VRRP) Feature Overview and Configuration Guide

This guide describes the Virtual Router Redundancy Protocol (VRRP) feature provided by the switch, and how to configure the switch to participate in a virtual router.

One function of a switch is to act as a gateway to the WAN for hosts on a LAN. On larger LANs, two or more switches may act as the gateway, and hosts use a dynamic routing protocol, such as RIP or OSPF, to determine the gateway switch to use as the next hop in order to reach a specific IP destination. However, there are a number of factors, such as administrative or processing overhead or even support for the protocols, which may make it undesirable to use a dynamic routing protocol. One alternative is to use static routing; however, if the statically configured first hop switch fails, the hosts on the LAN are unable to communicate with those on the WAN.

The Virtual Router Redundancy Protocol is defined in RFC 5798 (Virtual Router Redundancy Protocol (VRRP) Version 3 for IPv4 and IPv6). It provides a solution to the problem by combining two or more physical switches into a logical grouping called a virtual router (VR). The physical switches then operate together to provide a single logical gateway for hosts on the LAN.

A virtual router is configured as the host’s gateway and comprises a number of physical routers. The hosts can only see the virtual router so the number of physical routers that make up the virtual router is transparent. If physical routers in the virtual router fail, then traffic to and from the hosts will still be forwarded, so long as there is at least one functioning physical router, no configuration changes will be required by the hosts.