Hot Standby Routing Protocol

The protocol Hot Standby Router Protocol (HSRP) is a protocol owner of Cisco implemented on the Routeur S allowing a continuity of service. Each router using protocol HSRP belongs to a group. In this group, a router will be elected: that which will have the most raised priority. This router will be the active router of the group. Periodically, the routers exchange Hello messages to make sure that the routers of the group are still reachable. If the nominal router becomes inaccessible, or if the bond falls, another router will be elected: that which at the second highest priority. All the messages between the routers are exchanged by using the addresses multicast 224.0.0.2 (which corresponds to all the routers of the local bond) via UDP on the port 1985. The router having the most raised priority will comprise a virtual address IP as well as a virtual address MAC (0000.0c07.ac0A). These two parameters being added to the traditional configuration of the router. When the active router of the group becomes injoignable, another router takes over and thus recovers virtual address IP and the virtual MAC address.

Example: Are two routers (has and B) using protocol HSRP to provide a fault-tolerance. The router has will use address IP 192.168.0.1 with a mask of network of 255.255.255.0; the router B will use address IP 192.168.0.2 with a mask of network having for 255.255.255.0 value. We will define the router has like the router having the priority most raised then to add the router B to the group. Lastly, we will say that address IP 192.168.0.3 (255.255.255.0 mask) will be virtual address IP.

Router a:

interface Ethernet 0/0
IP address 192.168.0.1 255.255.255.0
standby 10 priority 100 preempt
standby 10 IP 192.168.0.3

Router b:

interface Ethernet 0/0
IP address 192.168.0.2 255.255.255.0
standby 10 priority 80 preempt
standby 10 IP 192.168.0.3

See too

RFC 2281 Cisco Hot Standby Router Protocol (HSRP)
Virtual Router Redundancy Protocol, RFC 2338

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