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New Member

Serial Interface (Looped)

I have a 1601 Router and the serial interface show Serial 0 is Up, Line Protocol is Up (Looped). The no loopback command has been entered and the interface is not in loopback and neither is my CSU/DSU. My circuit provider has ran loopbacks and the circuit looks good. I have also tried using three different CSU/DSU's. I feel that there is a loop up in one of the providers equipment, any other suggestions?

New Member

Re: Serial Interface (Looped)

First things first. There is a loop.

If you've eliminated your local CSU/DSU and your router, check the other side. Since you're up/up (looped), it means you're running HDLC. Check the device you're connected to on the remote end for loops on the router and the CSU/DSU.

The telco should also be able to tell you if traffic is reaching the CO at the far end. They should also be able to run a test from your CO to the remote CO and see if there's a loop there. Also, the telco should be able to open the circuit at different places in the path. If the circuit is open and you still get a loop, then the looped device is between you and where the circuit is open.

There's no easy way to do this if your remote side doesn't show any loops (check for remote-loopback on your remote CSU/DSU) because this points directly to the telco(s). They'll never admit when they're wrong unless you twist their arms behind their backs until they scream "Uncle!"

Good luck with this one.


New Member

Re: Serial Interface (Looped)

Sometimes you just have to do a 'shutdown' on the interface to reset it. Do 'shutdown' and then wait 5 seconds, then 'no shut'. I hope this helps.

Re: Serial Interface (Looped)


If you serial interface is detecting " interface looped " most likely your carried has not succesfully drop the loopback when they were doing testings, that is , assuming that you checked you CSU/DSU unit, but since you said that you've tried other CSU/DSU's is your carrier provider who needs to drop that loop at some point on that circuit..


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