9.2 Backup Interface Operations
9.2.1 Standby mode
When a backup interface is specified on a primary line, the backup interface is placed in standby mode. When in standby mode, the backup interface is effectively shut down until enabled. The backup route between the two company sites is not resolvable and it does not appear in the routing table. Example displays the output of the show interface dialer 1 command when it is run on the backup interface of the network displayed in Figure .

The primary link in Figure will be the only route appearing in the routing table. The branch office router continues to monitor the two following events:

  • Carrier detect
  • Keepalives

If the branch-office router does not receive either a keepalive or carrier-detect response, it assumes that the primary line is down and activates the backup link. If a primary line fails, the primary interface comes down and the backup is enabled. The routing table now reflects the backup route as the only resolvable route between the two company sites.

If a physical link is used as a backup to a primary connection, it is in standby mode and cannot be used as a link to another site.

In Figure , Branch Office A wants to back up its Frame Relay connection with ISDN Basic Rate Interface (BRI). Branch Office A also wants to use the same BRI interface as a DDR link to Branch Office B, however. If Branch Office A places the physical BRI link in standby mode, it is deactivated and does not activate until the primary line fails or reaches a specified threshold. Thus, the BRI link cannot be used to connect to Branch Office B.