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The Cisco innovation Hot Standby Router Protocol (HSRP) was introduced in IOS 10.0. HSRP brings fast re-routing technology to the desktop. HSRP enables a set of routers to work together to present the appearance of a single virtual router or default gateway to the hosts on a LAN. HSRP is particularly useful in environments where critical applications are running and fault-tolerant networks have been designed. By sharing an IP address and a MAC address two or more routers acting as one virtual router are able to seamlessly assume the routing responsibility in the case of a defined event or the unexpected failure. This enables hosts on a LAN to continue to forward IP packets to a consistent IP and MAC address enabling the changeover of devices doing the routing to be transparent to them and their sessions.
HSRP works by allowing an administrator to configure Hot Standby Groups to share responsibility for an IP address. Each router can be given a priority to enable an administrator to weight the prioritization of routers for active router selection. One of the routers in each group will be elected to be the active forwarder and one the stand-by router to stand ready to take over this functionality. This is done according to the router's configured priorities. The router with the highest priority wins and, in the case of a tie in priority, the greater value of their configured IP addresses will break the tie. Other routers in this group will monitor the active and stand-by routers' status to enable further fault tolerance. All HSRP routers participating in a standby group will watch for hello packets from the active and the standby routers. From the active router in the group they will all learn the hello and dead timer as well as the standby IP address to be shared, if these parameters are not explicitly configured on each individual router. If the active router becomes unavailable due to scheduled maintenance, power failure, or other reasons the stand-by can assume this functionality transparently within a few seconds. This will occur if the dead timer is reached, by missing three successive hello packets, and the standby router will promptly take over the virtual addresses, identity and responsibility.