3.3 Floating Static Routes
3.3.1 Configuring floating static routes
Static routing has disadvantages, one of which is that it cannot adapt to topology changes. However, you can configure static routing to have limited adaptability by creating floating static routes.

Floating static routes are static routes configured with an administrative distance value that is greater than that of the primary route (or routes). Essentially, floating static routes are fallback routes, or backup routes, that do not appear in the routing table until another route to the same destination fails. For example, assume that RTB is connected to network 10.0.0.0/8 via two different links, as shown in Figure . Also, assume that RTB's preferred route to network 10.0.0.0/8 is via RTC because that link has a higher bandwidth. This route is learned by RIP. RTB should use the slower link to 10.0.0.0/8 via RTA only if the primary route fails. The route to RTA is statically configured.

To statically configure RTB so that it will use the slower link to reach 10.0.0.0/8 after the RIP route fails, you must use a floating static route as shown:

RTB(config)#ip route 10.0.0.0 255.0.0.0 1.1.1.1 130

This ip route command includes an administrative distance of 130. Recall that static routes have a default administrative distance of 1 (see Figure ). To create a static route that will float (that is, wait for another route to fail before entering the routing table) you must manually set an administrative distance value. This value must be greater than the primary route's administrative distance value. In this example, the primary route is learned by RIP and thus has an administrative distance of 120. By configuring the static route with an administrative distance of 130, the static route will be less desirable than the primary route and the RIP route via RTC is always preferred. However, if the RIP route is lost, the floating static route takes its place in the routing table. Figure shows RTB's routing table with the RIP route and then, after the RIP route is lost, RTB's routing table with the floating static route.

The output in Figure includes output from the debug ip routing command, which details the loss of the primary route and the subsequent installation of the floating static route.

Floating static routes can be used in conjunction with other static routes to create a semi-adaptable static routing scheme. Consider this configuration :

RTZ(config)#ip route 0.0.0.0 0.0.0.0 s0
RTZ(config)#ip route 0.0.0.0 0.0.0.0 s1 5
RTZ(config)#ip route 4.0.0.0 255.0.0.0 s2
RTZ(config)#ip route 4.0.0.0 255.0.0.0 s3 5
RTZ(config)#ip route 4.0.0.0 255.0.0.0 s4 10

If RTZ is configured with these commands, it installs one route to 0.0.0.0/0 (using S0) and one route to 4.0.0.0/8 (using S2). If S0 becomes unavailable, RTZ will install the floating static route to 0.0.0.0/0 (using S1) into its routing table. If S2 fails, RTZ will fall back to using S3 to reach 4.0.0.0/8. Finally, if both S2 and S3 go down, RTZ will use the least desirable static route to 4.0.0.0/8, with an administrative distance 10.