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ISDN and backup interface


I have an ISDN BRI as the backup for T1 link.If I configure the BRI interface as the backup for T1 then in the Show ISDN Status layer 1 shows as deactivated.But if I configure a dialer interface as the backup for the T1 then layer is active and backup is working properly.What can be the problem when I use a physical interface as the back up for T1 line.

Can any one advise on this.



Re: ISDN and backup interface

If you used the backup command, that is normal behaviour. That command deactivates the BRI interface until the backup is initiated.

See the following link:

Cisco Employee

Re: ISDN and backup interface

There will be no problem..If you use interface (physical or dialer) with a backup command, that interface will be in "standby" mode untill the primary interface becomes down/down or up/down....You can verify what using "sh interface xxx"

With that router will disable the layer 1 for that physical interface..

But for dialer interface, its a virtual interface used for when you use "backup interface dialer x", only dialer interface will go in standby mode..BRI interface will remain as it is..

Its always a better idea to have backup command with dialer interface to avoid bri line with down/down mode which can trigger alarm to Some telco switches which telco people don't like that.

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