Cisco IOS Cookbook, 2nd Edition

Ensuring Proper Disconnection

Problem

You want to ensure that the dial backup line disconnects properly when the primary link recovers.

Solution

Sometimes funny things happen when the primary link comes back and the backup link has not yet disconnected. These problems are usually due to poor routing metrics, causing at least one of the routers to prefer the dial path, even if the primary is available. The easiest way to handle these problems is to use bandwidth commands to ensure that the primary is the better path:

Router1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#interface Serial0/0.1 point-to-point
Router1(config-subif)#bandwidth 56
Router1(config-subif)#exit
Router1(config)#interface BRI0/0
Router1(config-subif)#bandwidth 54
Router1(config-subif)#end
Router1#

Discussion

This example assumes that you have a Frame Relay connection using a 56 Kbps primary link, and an ISDN dial backup connection. The problem is that the default ISDN interface bandwidth is 64 Kbps. So, if both the primary and the backup are active at the same time, the routing protocol will see the backup link as the preferred path. As a result, there will always be interesting traffic flowing through the backup link, and the router will fail to disconnect the dialup session properly.

To address this problem, we have configured a bogus bandwidth on the BRI interface. This will ensure that if both primary and backup are active simultaneously, the primary will be the ...