The Physical Status of an E1 Channel Is Up and the Link Protocol Status Is Down, and a Line Loopback Is Detected

Fault Symptom

CPOS interfaces of a Huawei device and a non-Huawei device are connected through SDH transmission devices. The E1 channel of the CPOS interface runs PPP and does not have a loopback mode configured. Run the display interface serial command to view the interface status, and you can find that the physical status of the E1 channel is Up, the link protocol status is Down, and a loopback is detected.

Fault Analysis

The fault may be caused because the configuration of multiplexing path TUGs on the SDH transmission device does not correspond to the E1 channel number of the CPOS interface. As a result, signals are transmitted to the incorrect timeslot. Timeslots on both ends do not match. PPP negotiation fails and LCP cannot work properly. If a timeslot corresponds to an idle timeslot on the transmission device but the E1 channel of the correct timeslot on the transmission device is in loopback state, the system prompts a message, indicating that a line loopback is detected.

Troubleshooting Procedure

1. Run the display controller cpos e1 command to view the multiplexing path of the E1 channel. You can also obtain the multiplexing path of the E1 channel based on the E1 channel number of a CPOS interface and the result of Calculation of E1/T1 Path Sequence Number.

2. Check whether the configurations on the SDH transmission device correspond to the E1 multiplexing path obtained in step 1 and modify inconsistent entries.