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Cisco IOS Cookbook, 2nd Edition by Ian Brown, Kevin Dooley

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Introduction

First Hop Redundancy Protocol (FHRP) are a group of protocols that allow a router on a LAN segment to automatically take over if another one fails. They were developed to solve a common problem in shared networks such as Ethernet or Token Ring. The devices on this shared network segment are usually configured with a single default gateway address that points to the router that connects to the rest of the network. The problem is that even if there is a second router on the segment that is also capable of being the default gateway, the end devices don’t know about it. Therefore, if the first default gateway router fails, the network stops working. The three main First Hop Redundancy Protocols discussed in this chapter are HSRP Versions 1 and 2, VRRP and GLBP.

Many methods for addressing this problem have come and gone over the years. The most obvious and seriously flawed solution is to have the end users reconfigure the default gateway address in their workstations. This is a terrible solution for several reasons. There is a large chance of typographical errors. The conversion is slow and laborious, and often requires a reboot of the workstation. It relies on users noticing the problem in a timely manner, and it is unlikely that anybody will bother to change the address back when the original router recovers. And it also requires that there is a human handy to make the change, which is not always the case because many devices such as printers and servers don’t usually ...

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