Activating Dial Backup

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When a backup interface is specified on a primary line, the backup interface is placed in standby mode, as illustrated in the figure. Once in standby mode, the backup interface is effectively shut down until enabled. The backup route between the two company sites is not resolvable and does not appear in the routing table.

The primary link is the only route that appears in the routing table. The branch office router continues to monitor the line protocol of the primary interface or subinterface.

When the branch office router receives an indication that the primary interface is down, the backup interface is brought up. The amount of time that the device waits to bring up the backup interface is adjustable using the backup delay command. You can also configure the backup interface to go down (after a specified time) when the primary connection is restored.

The backup interface command is dependent on the router identifying that an interface is physically down. Because of this, the backup interface command is commonly used to back up ISDN BRI connections, asynchronous lines, and leased lines. This is because the interfaces to such connections go down when the link fails; therefore, the backup interface can quickly identify such failures. The backup interface approach may also be used for point-to-point Frame Relay subinterfaces. However, with Frame Relay, the main or multipoint interfaces can remain in an up/up state even if the PVC goes down. This could cause the router to fail to detect a down primary Frame Relay connection, and thereby fail to bring up the backup link.

A new development for end-to-end PVC management is a Cisco proprietary feature known as Frame Relay end-to-end keepalive. In Frame Relay end-to-end keepalive, keepalive packets are encapsulated in Frame Relay. This feature provides a status to verify that end-to-end communications are working and that traffic is getting through. This feature also allows a Cisco device to quickly detect that a link is down and enable the backup link.

This topic describes configuring a backup connection to delay engaging when the primary line fails, and delaying the shutdown of the backup interface after the primary interface is reenabled.

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