9.1 Dial Backup
9.1.2 Example of dial backup for link failure
In the Figure, interface serial 0 (S0) is the primary interface. You can see from the configuration displayed in the Figure that if the primary interface is down for 40 seconds, the backup interface (BRI0) will be activated. The secondary line will deactivate 20 seconds after the primary line is re-enabled. The example in the Figure illustrates only the commands to enable a backup. The interface must also be configured as needed (for DDR, Frame Relay, X.25, and so on).

An alternative to dial backup for link failure is a floating static route. A floating static route is a static route that has an administrative distance greater than the administrative distance of corresponding dynamic routes. Administrative distances can be configured on a static route, so that the static route is less desirable than a dynamic route. In this manner, the static route is not used when the dynamic route is available. If the dynamic route is lost, however, the static route can take over, and traffic can be sent through this alternative route. When this alternative route is provided by a DDR interface, DDR is used as a backup mechanism.