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New Member

giving loopback command in serial interface..


while going thru the documentation about trouble shooting serial interfaces,

I came across one situation as " serial x is up,line protocol is up(looped)".

The inference is that a loop exists in the ckt.The trobleshooting method says

that " use the sh runn-conf command to see any loopback command is there

in the interface configuration mode,and if it is there remove it".My question is that whether this "loopback command" is having the same effect as of

giving loopback in a modem or CSU/DSU.

And if not how can I use this command in trouble shooting?...

All your help will be thankfully appreciated...

  • Other Network Infrastructure Subjects

Re: giving loopback command in serial interface..

Since there has been no response to your post, it appears to be either too complex or too rare an issue for other forum members to assist you. If you don't get a suitable response to your post, you may wish to review our resources at the online Technical Assistance Center ( or speak with a TAC engineer. You can open a TAC case online at

If anyone else in the forum has some advice, please reply to this thread.

Thank you for posting.

New Member

Re: giving loopback command in serial interface..

When you loopback the interface through the CLI it is looped back in the hardware of the router. This is not the same as doing it at the CSU/DSU or beyond. When you loopback using the CLI you are not testing the serial cable, CSU/DSU, carrier line, etc. When an interface comes up and runs clean to a loop, it verifies all equipment to the loop point. This is a good troubleshooting tool. Move your loop point in either direction until you isolate the faulty part.