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Backup Overview

As you can see from Figure 10-1, a backup interface can be either a physical interface, such as an ISDN BRI interface, or an interface assigned to a dialer pool. When a backup interface is specified, it remains in standby mode until activated. When the interface is in standby mode, it remains idle, and the backup route between the routers does not appear in the routing table.

As soon as a backup interface is configured, the router monitors the following on the primary link:

• Carrier detect signal

• Keepalives

If the router misses either a carrier detect signal or a keepalive, the primary link is assumed to be down, and the route is withdrawn from the routing table. When this happens, the backup interface is activated, and the backup route appears in the routing table.

Routers can also be configured to bring up a backup interface when the load on the primary interface meets or exceeds a certain limit. This limit can be configured.

Backup interfaces can be activated when certain events occur. Backup interfaces can include

• Serial interfaces

• ISDN interfaces

• Asynchronous interfaces

• Dialer pools

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