Router redundancy is the concept of creating high network availability by using a group of routers to support a shared IP address. To implement this concept, you create an HSRP group -- a group of two or more routers that is managed by HSRP (Hot Standby Routing Protocol). HSRP is often used for a gateway router. It creates the appearance of a single virtual router or default gateway to the hosts on a LAN.
HSRP uses a virtual Media Access Control (MAC) address and a virtual IP address that are shared among a group of configured routers. It allows two or more routers to use the MAC address and IP network address of a virtual router. The virtual router does not exist; it represents the common target for routers that are configured to provide backup to each other.
HSRP selects one of the routers to be the active router -- the router of choice for routing packets -- and another to be the standby router, which assumes control of the group MAC address and IP address if the active router fails.
To assign a Catalyst 3550 multilayer switch to an HSRP group (or to remove it from an HSRP group), choose Device > Router Redundancy and use the Router Redundancy window.
"Configuring HSRP," Catalyst 3550 Multilayer Switch Software Configuration Guide