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New Member

How to use ISDN both for incoming call and for backup


Cisco 3600 with 8 BRI module and serial line connection to the Central site. The BRI's are accepting incoming calls. I want to configure one of the BRI's to call the central site in case te serial connection goes down. I know it can be done by dedicating one of the BRI's to act as backup either using dailer watch, backup interface or floating static, but I would like to know is it possible to congigure the following situation:

All the BRI's are used at certain moment for incoming calls and they are all connected. The serial line fails. One of the BRI's drops its dialin connection and starts backup procedure.

Is this possible and are there any articles on this?



New Member

Re: How to use ISDN both for incoming call and for backup

Hi Velimir,

How about configuring one of the BRI's to be a member of two different dialer pools, and make sure PPP multilink is not used. So if one B channel is in use, the other can be free to use as a backup? You could apply this to 2 or 3 physical ports and group them to give you some bandwidth.

Actually, you'll need to use rotary groups I think.

Theoretically it should work although I haven't tried it.

Just a thought....



Re: How to use ISDN both for incoming call and for backup

The simplist approach would be to configure a dialer map on a rotary-group for your backup call (all the BRI's are a member of this rotary-group), and then configure a very small (maybe 5 seconds) fast-idle timer. What will then happen is when we have interesting traffic that will cause dialing (due to the serial going down, for example), if all the links are currently active and in use we will *decrease* the idle timer from it's original setting to the fast-idle setting, such that any link(s) that are idle for this shorter duration will be disconnected, thus freeing up a link(s) for dialout.

This *may* cause more links than you would like to be disconnected; you will want to play with this to see if it meets your needs.

New Member

Re: How to use ISDN both for incoming call and for backup

Thaks I will look into what you suggest


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