Hot Standby Router Protocol

Increasing the availability of of gateways

The Hot Standby Router Protocol ( HSRP ) is a method for increasing the availability of key gateways in local networks with redundant routers.

The proprietary HSRP has been developed by Cisco. Today, however, it is also available in many routers from other manufacturers. The HSRP is designed to address one of the major problems in the reliability of data paths, the availability of the so-called default gateways in local networks.

When routing of data packets through networks the failure of legs is counteracted on the way to the target by the dynamic routing. However, if from the first router on a path, this error can not be solved by dynamic routing, as a host usually has only a static default gateway and usually only one supported. Due to a failure of the first router, the default gateway, the affected hosts of the communication are cut off with the outside world, even if still active routers are on the same subnet.

At this point, the HSRP is where. Several physical routers are combined into a logical group. This group of routers in the network presents itself now as a logical router.

For this purpose, the logical router, a virtual IP address and one virtual MAC address is assigned. The virtual MAC address consists of the prefix 00-00 - 0c -07 -ac and the HSRP group number in hexadecimal notation. One of the routers within the group is defined as the primary router, it binds on the virtual MAC and virtual IP to its network interface and informed at regular intervals (default: 3 seconds) the other routers of the group, which act as secondary router by means of a multicast message to the destination address 224.0.0.2, port UDP 1985.

If the primary router and remain the multicast packets for longer than the time of the "hold -down timers " (default: 10 seconds ), so the virtual IP address and virtual MAC address is immediately on one of the secondary router transmitted, the becoming the new primary router. The MAC and the IP address are transferred, so that the affected hosts do not need to update their ARP cache. The risk of failure of the first router on the route can be reduced.