CISCO IOS in a Nutshell

Hot Standby Routing Protocol (HSRP)

In this book, several examples dealt with creating backup links for use in case some other link fails. But routers themselves fail—so how do you implement backup routers? You might think that this shouldn’t be a problem: after all, if you have two routers connecting your site to the external world, and one fails, your own routing protocols should eventually route around the failure. However, it’s not that simple. If the individual hosts on your network aren’t running some sort of routing protocol (such as RIP) or router discovery protocol (such as IRDP), they’ll never find out about the failure. And in most cases, you don’t want to be running a routing protocol on individual hosts; you want to set up each host with a simple default route and leave it at that. Furthermore, when a router fails, it can take some time for a protocol such as RIP to converge on a new route.

That’s the overall picture. More concretely, the top illustration in Figure 12-1 shows a larger network comprising two smaller networks. There are two routers, both connected to the external gateway. Let’s assume that you want to configure the hosts on Network 1 and Network 2 with static default routes. To which router should these default routes point? All the hosts on both networks can reach either router, but each host can have only one default route. What happens if the router a particular host is using fails? How does that host find out about the other router, aside from an ...