HSRP defines a standby group of routers, with one router as the active
router. HSRP provides gateway redundancy by sharing IP and MAC addresses
between redundant gateways. The protocol consists of a virtual MAC address and
IP address that are shared between two routers that belong to the same HSRP
standby group.
HSRP can
optionally monitor both LAN and serial interfaces via a multicast protocol.

An
HSRP standby group comprises these entities:
- One active router
- One standby router
- One virtual router
- Other routers