4000 and 2500 Series Routers ISDN Problem

Unanswered Question
Mar 11th, 2008


My compaany has two locations. We have had them connected via ISDN. It has been working for the past 4 years, but all of the sudden (last Friday) the connection went down in the middle of the night. We have been troubleshooting for days with absolutely no luck !!! Our Telco Company (Telus) says that the ISDN line tests fine on both sides.

Our Setup is as follows:


Cisco 4000 Router--> Adtran ISU128

-----(over the ISDN line)------>


Adtran ISU128---> Cisco 2501 Router.

We have always had a Multilink PPP (MPPP) Link across this network.

As near as I can tell, I think that the configuration of one or both of the routers has been compromised. The 2501 seems to be acting as the dailer in our case. I have double checked the modems on both sides, and their config. looks correct.

I was wondering if there is anyone out there that knows the correct setup and configuration for the two routers (4000 and 2501) so we can re-input the config and start from scratch.

Any and all suggestions would be welcome.

Thank you in advance.


I have this problem too.
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shillyar Tue, 03/11/2008 - 16:50


please run debug isdn q931 on both sides place a call and gather the logs and debugs. Also sh isdn status and make sure L1 and L2 are up and multiple frames established.

As for the conf this is a pretty simple setup that works reliable.

Thanks Steve

Goutam Sanyal Tue, 03/11/2008 - 22:03


STEVE HILLYARD's post is very helpful. With the addition I'd like to request you to check the ISDN line (by checking the voltage of the last mile pair, provided by telco.). It should in between 95-105 V DC. If you are getting the voltage then L1 and L2 will show the active status. Also try to dial any ph. no and check the call is establishing or not by “sh isdn his”




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