HSRP: Hot Standby Router Protocol
HSRP is a Cisco proprietary protocol that allows multiple Routers to act as a single gateway for a network. Once HSRP is enabled all of the Routers in an HSRP group send out hello messages every 3 seconds to tell other routers in the group that they exist and what their current state is.
Based on a priority value the Routers elect an active router to be the gateway router for the network. In addition to Hello messages routers in the group can send out Coup messages if their priority is higher than the current active router – forcing the current active router to resign (by sending out resign packets) and that dethroned router enters the speak state. From there it will either go into the listen state or the standby state.
Routers in an HSRP group go through the following states before they become an active Router: