Cisco loop testing must be done by Encapsulation HDLC

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Loop testing is common method for isolating and troubleshooting circuit related issue or rule out hardware issue for serial or SONET links. I guess many of T1s or below and multi-link connections are using either frame-relay encapsulation or ppp. There are common mistake that tester didn’t change encapsulation HDLC while loop was provided by other end. 

Especially, using Cisco gear, all loop testing must be done by Encapsulation HDLC protocol.

It doesn’t matter it is soft-loop or hard-loop, Cisco gear must change encapsulation HDLC when you run to the loop in place on the circuit.

Don’t forget copy and past original configuration, especially Frame-relay encapsulation is being used. B/C when you change encapsulation HDLC, Cisco gear will delete entire Frame-relay encapsulation. If you have static route statement using interface name to route, it will be gone by changing encapsulation HDLC for testing.

Sometime, Telco tester reported frame errors or bit errors Cisco router which has remaining encapsulation ppp. Most of case, they saw improper signal from Cisco router under encapsulation ppp. Telco tester must check encapsulation HDLC on Cisco router before starting analysis circuit.

It is very important tips also, it apply OC3 SONET environment as well.

I hope this is informative for you.