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Hot Standby Router Protocol

In computer networking, the Hot Standby Routing Protocol (HSRP) is a Cisco proprietary redundancy protocol for establishing a fault-tolerant default gateway, and has been described in detail in RFC 2281.

The protocol establishes a framework between network routers in order to achieve default gateway failover if the primary gateway becomes inaccessible, in close association with a rapid-converging routing protocol like EIGRP or OSPF. HSRP routers send multicast Hello messages to other routers to notify them of their priorities (which router is preferred) and current status (Active or Standby).

The primary router with the highest configured priority will act as a virtual router with a pre-defined gateway IP address and will respond to the ARP / ND request from machines connected to the LAN with a virtual MAC address. If the primary router should fail, the router with the next-highest priority would take over the gateway IP address and answer ARP requests with the same MAC address, thus achieving transparent default gateway failover.

HSRP version IP Protocol Group address UDP Port Virtual MAC address range
1 IPv4 (all routers) 1985 00:00:0c:07:ac:XX
2 IPv4 (HSRP) 1985 00:00:0c:9f:fX:XX
IPv6 ff02::66 2029 00:05:73:a0:0X:XX

In the virtual MAC address, Xs represent the group ID in hex.

HSRP is not a routing protocol as it does not advertise IP routes or affect the routing table in any way.

HSRP has the ability to trigger a failover if one or more interfaces on the router go down. This can be useful for dual branch routers each with a single link back to the gateway. If the link of the primary router goes down, the backup router will take over the primary functionality and thus retain connectivity to the gateway.

Source: Wikipedia, Google