Problems: Will a bad CSU/DSU in a 2621 cause a line to be looped

[aseamans]

Ok i have a question for all you cisco experts out there. I am kinda new at the whole router thing. We had a site's T1 line that goes into a 2621 become in a looped state when we came in in the morning. When we called for maintainence on the circuit they found a foreign loop and dropped it. when they did the whole circuit went down. They then said it was our CSU/DSU. However I could loopback through the csu/dsu via the DTE command. The circuit will come up sometimes then just drop again. With a loopback set on something I would venture to guess someone did some testing on the line and did something they were not supposed to and won't admit to it. I did reboot the router once and the line did come up but I think it was coincidence. I have a remote alarm and loss of frame errors on the serial port. Anyone have any idea what is going on?

[g.hillabrant]

It is possible that your csu/dsu is dying. The smart jack at your site could have an automatic loopback feature. Which means if the DS1 signal from your router fails the NID automatically loops itself to prevent alarms from traveling up the telco chain. Telcos dont care if your network is up or not as long as it's not their equipement that's bringing you down, thats why the auto loopback feature. If the problem was on the transmit side of the csu/dsu you would be able to loop it up and run to it, the csu just wouldn't forward the data to the NID, hence the LOF and sending the network into loopback. This is just a blind guess, I would contact your T1 provider and have them test the circuit.

Greg

[aseamans]

what is wierd about the whole thing is now the circuit has been up all day without any complaints. Yesterday I could loopback through the csu/dsu just fine. I think someone at the local bell was doing some testing on the wrong line and messed something up. That is what I think it was. I did contact the T1 provider and everything came up clean to the smartjack is what they said. However could someone have changed the frame type by mistake on the circuit and did not realize it? After all they put a fiber backbone for a large amount of people for the area in our space. : )

[g.hillabrant]

Its very possible that the service provider accidently looped your circuit. I know I've done it many times, and will do it again. When you have 15,000 T1 lined up side by side it's not hard to plug you're net monitoring equipement into the wrong jack. Changing the frame type is a lot more unlikely, but also still probable.

greg

[aseamans]

it just got more interesting today. I want to make sure that I am right and that is the only reason I keep on this problem that has been "mysteriously" resolved.

I found out today that another client of ours has their own Fractional T-1 in our space. Now the T-1 is actually a frame relay circuit. It appears that they went down today. It appears that they were scheduled to drop the old path to the circuit and put it on a new one and someone messed up or something like that. They were clueless and disorganized. With this info it makes my theory hold more water than before. Last week we had ours go down and everything I did pointed to a wrong frame type. This week a client's frame went down. Coincedence? I think not.

[gmiller]

A bad CSU/DSU won't cause the line to be looped , What I have found is during the night your circuit may have taken a hit and your vendor starts testing the circuit,but fogets to remove the looped.

Any alarms on the CSU/DSU ? if so have your vendor test the circuit for opening .

Is the V.35 cable ok ? if not replace it

What the name brand of CSU/DSU ?