7 Replies Latest reply: Oct 7, 2010 12:58 PM by Alamedian RSS

    Loop on a Circuit

    Dada Khalander

      Hi All,


      What is a loop on a T1/T3 circuit?

      What is hard loop and what is soft loop and how can we see it?

      And when can we say that the loop is clean on the circuit?




      Thanks & Regards,

      S. Dada Khalander.

        • 1. Re: Loop on a Circuit
          Paul Stewart  -  CCIE Security

          A loop on a circuit is when the tx is connected to the rx.  This can be anywhere in the circuit.  It can be a hard loop with the actual wires connected from tx to rx (with a t1 pin 1 to pin 4, pin 2 to pin 5, pin 4 to pin 1 and pin 5 to pin 2).  Also, it can be a soft loop in which one of the devices along the way place a loop in the circuit through configuration.  We can send traffic through the loop to see if we are receiving any errors in that area of the circuit.  The statement, "When can we say the loop is clean?", needs a bit more explanation.  Maybe someone is talking about the circuit running clean, or maybe they are talking about the loops being removed from the circuit.  Again, we can send traffic through the loop to see that an area of the circuit is clean.  If the question is how can we know when we have no loops in the circuit, it is when we no longer see our own keepalives.

          • 2. Re: Loop on a Circuit
            Dada Khalander

            Hi Paul,


            Thanks for the answer. But how can we test the soft loop on the circuit. Can you please share the configuration commands?




            Thanks & Regards,

            S. Dada Khalander.

            • 3. Re: Loop on a Circuit
              Dada Khalander

              Any updates on this request would be greatly appreciated...




              Thanks & Regards,

              S. Dada Khalander.

              • 4. Re: Loop on a Circuit

                Please see link:




                Again as said above, if you have no errors on the looped line counting up over an hour or 24 or 72 hours or for example during a bad flood of rain water then the line tests OK.

                • 5. Re: Loop on a Circuit

                  Ideally you should have a process or a number defined within the Policies and Procedures, or network standards, to define what you call "clean".


                  Often times, a circuit that runs clean for an hour is considered clean, then put into monitoring for 24 hours.  If after 24 hours the circuit is still clean, you're good to go!  However, problems can be intermittent, due to third party interference, weather patterns, other environmental factors, etc.


                  Unfortunately learning how to really troubleshoot circuits is learned by doing, rather than reading about it.  It all depends on the transport method used.  Frame relay may be tested differently than an ethernet shot, which may be different than a direct DWDM shot, which is different than microwave.

                  • 6. Re: Loop on a Circuit

                    I'm not sure I understand. Do you want to test the loop (to see if the circuit is looped)? Or do you want to test the circuit.


                    To test the circuit you run what is called a BERT test. Stands for Bit Error Rate Test. You attache a BERT tester to a looped link and send patterns of data into the circuit. if the BERT tester receives the same pattern that it sent, then the link is clean or error free.



                    • 7. Re: Loop on a Circuit

                      Mitch, as you said - you run the test on a looped circuit.  I think the OP is just starting to learn about circuit testing hence the confusion in terms.