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New Member

ISDN Backup for Serial Line

I'm currently trying to figure out a way to test my ISDN backup for my Frame Relay sub-interfaces.

One of my Frame-Relay cards is split into four sub-interfaces, each sub-I/F has a dedicated standby BRI in standby mode.

Does anyone know a way I can test the fail-over without removing my serial cable.


New Member

Re: ISDN Backup for Serial Line

three ways

1, shut down your main serial interface, NOTE, not sub-interface

2, change the encapsulation frame relay to hdlc or others

3, shut down your FR switch


Re: ISDN Backup for Serial Line

just remove the backup interface command it the isdn should dial . if it does not you can ping it up by pinging the ip address in your dialer map.

New Member

Re: ISDN Backup for Serial Line

Hi Padraig,

Good Day! :-)

Here is an excerpt on how you could test the backup link for your frame-relay connection.

Testing the Backup Link

The backup interface requires that the primary interface protocol be down before the backup interface is even brought out of standby mode. Hence, it is not possible to test the backup link without physically brining the primary link down. The options are:


Use the shutdown command to turn off the primary interface. However, do not use the shutdown command on the router with the backup interface command configured. This will not cause the Cisco IOS software to dial the backup link. You can activate the backup by shutting down the primary interface on the router that does not have the backup interface command. In our scenario, the backup interface command is configured on maui-soho-01 (1600). Therefore, performing a shutdown command on the primary interface of maui-nas-05 (3640) will activate the backup link.


Physically bring down the primary connection by unplugging cables or some equivalent method in order to bring up the backup interface.

You may also refer to this link for more information.

Please let me know if this are the information that you are looking for.

Best Regards,

Ansley R. Verzosa

New Member

Re: ISDN Backup for Serial Line

Hi Ansley,

Thanks for your help.

This should sort me out.


Re: ISDN Backup for Serial Line

Be aware that "backup interface" is a problematic method of backing up frame relay links, above and beyond the challenge of nondisruptive testing. Plan on unscheduled downtime if you don't have "end-to-end keepalives" configured on your frame relay PVCs.

Seriously consider using DDR or dialer watch for your ISDN backup configuration. You'll save yourself a lot of pain and grief in the long run.

Vincent C Jones