Hot Standby Router Protocol (HSRP)

Hot Standby Router Protocol (HSRP) was developed by Cisco Systems as a way of providing fault tolerance for routed internetworks. Normally, when a router goes down, routing protocols communicate this fact to all the routers on the network, which then reconfigure their routing tables to select alternate routes to avoid the downed router. The problem is that the process of convergence, the updating of all routing tables on the network, is slow and can take some time to complete. HSRP was designed to work around this problem of slow convergence by allowing a standby router take over from a router that has gone down and fill its role in a manner completely transparent to hosts on the network.

HSRP works by creating virtual groups that consist of two routers, an active router and a standby router. HSRP creates one virtual router in place of each pair of actual routers, and hosts need only to have their gateway address pointed to the virtual router. When a host forwards an Internet Protocol (IP) packet to its default gateway (the virtual router), it is actually handled by the active router. Should the active router go down, the standby router steps in and continues to forward packets it receives. To IP hosts on the network, the virtual router acts like a real router and everything works transparently whether the active or standby router actually does the forwarding.