Hot Standby Router Protocol (HSRP)
HSRP is a Cisco proprietary protocol.
With HSRP, two or more devices support a virtual router with a fictitious
MAC address and unique IP address. Hosts use this IP address as their
default gateway, and the MAC address for the Layer 2 header. The virtual
router’s MAC address is 0000.0c07.ACxx, where xx is the HSRP group.
Multiple groups (virtual routers) are allowed.
The Active router forwards traffic. The Standby is backup. The standby
monitors periodic hellos (multicast to 224.0.0.2, UDP port 1985) to detect a
failure of the active router. On failure, the standby device starts answering
messages sent to the IP and MAC addresses of the virtual router.
The active router is chosen because it has the highest HSRP priority (default
priority is 100). In case of a tie, the router with the highest configured IP
address wins the election. A new router with a higher priority does not cause
an election unless it is configured to preempt—that is, take over from a lower
priority router. Configuring a router to preempt also insures that the highest
priority router regains its active status if it goes down but then comes back
online again.
Interface tracking reduces the active router’s priority if a specified circuit is
down. This allows the standby router to take over even though the active
router is still up.