AR 18-31 is a Low-End Router with ADSL WAN interface which can be backup by another ISDN WAN backup interface.
This case is a reference that points the correct configuration of the router when we want to use the backup ISDN interface to ensure a reliable connection to WAN network.
The configuration in this case for the router(the one that is seen in some of the manuals is the following) is in the attached .txt file.
Everything looks ok on this configuration, ADSL dialer is dialing when the cable is inserted in the ADSL interface, the ADSL connection is working well, but the problem apears when we unplugg the ADSL cable and the ISDN connection should backup the ADSL.
The handling process is to use the second configuration in the .txt file.
In this case the BRI interface has smaller preference and when this interface will exit the standby status will go up and the route "0.0.0.0/0 STATIC 200 0 0.0.0.0 Dialer10" will be replaced with "0.0.0.0/0 STATIC 60 0 0.0.0.0 BRI3/0".
The router will get the ip address for the backup interface and the WAN connection will be succesfully backup.