Building a resilient network using HSRP/VRRP

A group of key servers losing network connectivity can lead to a real bad day. You can improve your network routing resiliency by adding hot-standby routers and HSRP (Hot Standby Routing Protocol) or use a similar method such as VRRP (Virtual Router Redundancy Protocol) for non-Cisco devices. HSRP provides a straightforward approach by having two physical routers accessible by any given switch. It’s a great approach to improving the reliability of your key equipment. Here are some quick basics on how it works:

1. Two routers share a virtual IP address that is used as the gateway IP.
2. A primary and secondary router are designated with each given a priority number. The primary router priority number is higher than the secondary router.
3. The primary router sends a Hello packet to the secondary on a timed basis. If this packet is not received then the secondary becomes the primary. Very low packet loss occurs during a failover and most transmissions using TCP are completed seamlessly due to the re-try mechanism. The re-try mechanism can be controlled by adjusting the length of the time between Hello packets.
4. Once back online, the primary router sends out a notification that includes its priority number. The router with the highest priority number becomes the primary.

This process can be used in groups with multiple routers offering to become the primary router in the event of a failure. Some devices even support using the same interface for multiple groups. This can be helpful when cross connecting multiple departments and to minimize the amount of duplicate hardware necessary.

The following is a sample set of IOS commands necessary to implement HSRP.

Router> enable
Router# configure terminal
Router(config)# interface ethernet 0
Router(config-if)# ip address 172.16.6.5
255.255.255.0
Router(config-if)# standby 1 ip 172.16.6.100
Router(config-if)# end
Router# show standby
Router# show standby ethernet 0

For more information review the Cisco documentation.