Definition
Hot Standby Router Protocol (HSRP)
Hot Standby Router Protocol (HSRP) is a layer 3 redundancy protocol used in networks to ensure the availability of a default gateway.
It is the Cisco-proprietary equivalent of the open Virtual Router Redundancy Protocol.
Routers
Active Router
A single router is elected as active router which handles the packet forwarding to the default gateway IP address, which is configured the HSRP virtual IP address.
Standby Router
The standby router takes over if the active router fails.
Election
Each router has a priority which is communicated to the other via Hello messages. The router with the higher priority becomes the Active router.
If there is a tie (both have the same priority), the router with the highest IP address becomes active while the router with the second highest IP address becomes standby.