Tip

Configure backup static routes

If somehow a router's dynamic protocol gets a problem, it's a good idea to have a backup static route. That way, even when the dynamic protocol messes up, the router will still be functioning. This tip, excerpted from Managing IP Networks with Cisco Routers

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, by Scott M. Ballew, tells you why and how.

You may find it useful to have a static route that is in place as a backup to your dynamic routing protocol. For example, you might want such a route on a remote router (perhaps at a branch office) to which you do not have convenient access. This route would prevent you from losing contact with the router if you make a mistake while configuring it from your office. By having a backup static route, you can take advantage of dynamic routing wile knowing that the backup static route will allow you to fix mistakes without traveling to the router in question.

The trouble with a static route is that, on most routers, static routes normally supersede any route learned by a dynamic protocol. We only want the static route to take effect in an emergency -- for example, because we've screwed up the dynamic protocol with a configuration error. In other words, we want the dynamic protocol to take precedence, but we don't want to be without a route when the dynamic protocol doesn't have one.

The way to allow a static route to be superseded by a dynamic routing protocol varies from router to router. In the case of Cisco IOS, you assign an administrative distance to the static route that makes it less preferable than your dynamic routing protocol. Other routers will probably have a similar mechanism that lets you select routes learned from one routing protocol over another; this mechanism will usually treat static routes as another routing protocol. To set the administrative distance for our backup static routes, we need to know a few of the many default administrative distances that Cisco has assigned. The important ones for our example are:

Connected interface0
Static route1
EIGRP summary5
EIGRP internal90
IGRP100
OSPF110
RIP120
EIGRP external170
Unknown255

A lower administrative distance is preferred over a higher distance, and a routing source with an administrative distance of 255 is never used. So, in each of our examples, I need only increase the administrative distance of the static route to the network management subnet to something higher than my dynamic protocol. To do so, I append the desired distances to the end of the static route statement.



This was first published in November 2001

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