The Host Standby Router Protocol (HSRP).
The Host Standby Router Protocol (HSRP) is a Cisco proprietary protocol, as detailed in RFC 2281. HSRP provides gateway redundancy by sharing IP and MAC addresses between redundant gateways. The protocol consists of virtual MAC and IP addresses that are shared between two or more routers that belong to the same HSRP group.
How HSRP works.
HSRP can be configured on a cisco router as a “virtual” router to be used in the routing of packets when the active router interface fails. Basically, what HSRP does is to stand in as a backup router, standing by for when the active router gateway interface fails.
This “virtual” router is configured with a single IP address (layer 3) and MAC address (layer 2) which is shared among two or more router on a LAN segment.
The IP address of the virtual router is configured as the default gateway for the clients on a specific IP segment. When frames are sent from the clients to the default gateway, the clients will use ARP to resolve the MAC address that is associated with the IP address of the default gateway. The ARP then replies with the MAC address of the virtual router. Frames that are sent to the MAC address of the virtual router can then be physically processed by any active or standby router that is part of that virtual router group.
HSRP can be classified as a redundancy protocol that provide a mechanism for determining which router should take the active role in forwarding traffic and determining when that role must be taken over by a standby router.
Active router: The router that is currently forwarding packets for the virtual router
Standby router: The primary backup router
Standby group: The set of routers participating in HSRP that jointly emulate a virtual router
The primary function of the HSRP standby router (virtual) is to monitor the functioning status of the HSRP group and to quickly assume packet-forwarding responsibility if the active router fails.
These are the steps that take place when a router or Layer-3 device (switch) fails:
1. The standby router stops receiving hello messages from the forwarding router.
2. The standby router assumes the role of the forwarding router.
3. Because the new forwarding router (standby router) assumes both the IP and MAC addresses of the virtual router, the connected network devices see no disruption in service.