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Community Member

Trouble isolation with hard loop question

Has anyone successfully used a female loopback plug on a cable that is plugged in demarc to test from router through csu and extended demarc and then looped back. Cisco lists it as an option for this kind of troubeleshooting, but I failed today when I tried it. I simply changed encap to hdlc and assigned an ip to my unassigned physical interface. I was expecting the interface to read up/up (looped) when the plug was used, but it did not in this case. I backed up to csu and still no luck. My wire tech did not have a male loopback plug or I would have had him stick that in the s0 to see if it bought it up. I have not really used this technique and wanted to see if others had. Thanks in advance for the answers.

5 REPLIES
Silver

Re: Trouble isolation with hard loop question

I am not really sure what you mean by female loopback plug, however a loopback plug can be performed from any point in the line that allows for a loopback. If you are talking about a T-1 loopback from your Demarc location then you can generate a loopback facing your equipment by just using an RJ-45 and hardwire wires 1 to 4 and 2 to 5. Or if using a DB-15 cable you can use a loopback as long as you make it correctly and it is facing the correct side that you want to test... However to get beyond your question, You may have a cabling issue from the Demarc to your CSU/router... When you have the circuit normalized, what are the control signals you see from the CSU and from the router Show Interface serial command???

Community Member

Re: Trouble isolation with hard loop question

A female loopback plug allows you to plug the end of a cable in to the plug so as to test the cable span, as opposed to a male that will plug directecly in a device or jack. From a wiring perspective, it is exactly as you describe. In terms of the control data from my csu, I cant give you that now because I left this ckt with the csu plugged directly in the demarc for testing purposes, and the router is not inline. Thanks for the info here.

Gold

Re: Trouble isolation with hard loop question

Never thought about it being called a female loop plug I alway just put a male one in a coupler block and plugged that in the end of the cable.

As long as you have the wire crossed correctly I guess it doesn't matter how you do it.

You know you can alway cut up a lan cable and twist the right pairs wires together to get a loop plug when you are really desperate.

Community Member

Re: Trouble isolation with hard loop question

Yes, I have to do that from time to time (have a tech build there own hard loop). For me, I work for a large provider and I am used to loopback testing from our DAX's toward CPE all the time. Where I am out of my element is doing it in the other direction, using the router for telemetry of the test, toward the LEC. Have you down much of that?

Gold

Re: Trouble isolation with hard loop question

We use the router to test the lines kinda depends on if we have another access to the router. Since in building cabling and the router interfaces are blamed all the time it is common for the provider to give me a loop from the demarc toward the router to test.

If we find errors then we insert hard loops onto the cables toward the router and work back.

Now what you see on the router will depend on a lot of stuff. Most the time I reconfigure the interfaces back to HDLC and put IP on. This allows me to run extended ping test to generate traffic. You in some cases may need to set your clock to internal if you see frame slips.

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