Callback failure when using two carriers - ISDN PRI

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Nov 28th, 2003
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A customer has a 3660 with two ISDN PRI interfaces with different carriers, hence two different number ranges. When a remote user dials in and requests callback, the callback fails half the time. If just one PRI is used the callback works, whichever carrier is used. I think the problem is due to the inherent security of ISDN.

If remote user dials in from "1234" to the router on "6789" and the callback goes back out on the same carrier, then the return call is from "6789" to "1234" and the call succeeds.

If the next call is from "1234" to "6789", the router uses round robin and selects the other PRI to make the callback. The return call will be from, say, "4567" to "1234" and because the numbers differ, the call is rejected.

I have got this right and if so, is there any way round it.


Colin Spencer

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hbaerten Sun, 11/30/2003 - 23:17
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Hi Colin,

I have no experience with this particular setup but it doesn't seem logical to me.

Have you tried "debug isdn q931" to see how the call gets set up and why it may be failing?

Also, how is callback implemented? Is it ppp callback or isdn (dialer) callback?

Is the remote a cisco or a PC? etc.



cospencer Mon, 12/01/2003 - 01:57
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Remote users are typically dialing in using laptops on pstn. They enter their number for callback and the router dails back. The customer isn't able to do any testing for a few weeks so I'm unable to go to site and run debugs. I'll investigate further when I go to site.

Thanks for the input though.




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