Hot Standby Router Protocol (HSRP)
Hot Standby Router Protocol (HSRP) is a first hop redundancy protocol that is used to establish a fault tolerant default gateway. It is a Cisco proprietary protocol, but is also used by other vendors, as it has been defined in RFC 2281.
HSRP establishes an association between two or more gateways. Gateways can be any type of Layer 3 interface, including physical interfaces on routers, or SVI interfaces on Layer 3 switches. A virtual IP address and MAC address are created and shared by the participating gateways.
The active gateway adopts those virtual addresses and actively operates as the gateway, receiving and forwarding packets from hosts. In the event of a failure of the active gateway, alternative physical gateways adopt the virtual addresses and continue forwarding traffic from hosts.