First Hop Redundancy Protocol (FHRP)

Gateway Redundancy

Importance of Redundancy

Redundancy is a large part of enterprise network design. When designing enterprise networks, single points of network failure should be avoided whenever possible, particularly when network hardware is involved. Layer 2 redundancy can be accomplished quite easily by connecting duplicate backbone links. The spanning-tree algorithm used in most bridges and switches today by default will disable one of the links, leaving it in a suspended state. In case of a failure, the suspended link is enabled, and service is restored. At Layer 3, it is possible to have gateways and nodes share routing information. AppleTalk and IPX behave this way by default.

First Hop Redundancy Protocol (FHRP) is a hop redundancy protocol that is designed to provide redundancy to the gateway router within the organization’s network using a virtual IP address and virtual MAC address.

The network in the picture above is simple.
I have one computer connected to a switch. In the middle you’ll find two multilayer switches (SW1 and SW2) that both have an IP address that could be used as the default gateway for the computer. Behind SW1 and SW2 there’s a router that is connected to the Internet.

Which gateway should we configure on the computer? SW1 or SW2? You can only configure a one gateway after all…

If we pick SW1 and it crashes, the computer won’t be able to get out of its own subnet because it only knows about one default gateway. To solve this problem, we will create a virtual gateway:
Between SW1 and SW2 we’ll create a virtual gateway with its own IP address, in my example this is 192.168.1.3.

The computer will use 192.168.1.3 as its default gateway. One of the switches will be the active gateway and in case it fails the other one will take over.

There are three different protocols than can create a virtual gateway (implement FHRP):

• HSRP (Hot Standby Routing Protocol)
• VRRP (Virtual Router Redundancy Protocol)
• GLBP (Gateway Load Balancing Protocol)