3Com MSR 50 Series Configuration Manual page 547

Perform the display interface serial command to check information on interface
status. It shows that the physical state of the interface is UP, but the link protocol is
DOWN; and loopback, though not configured, is detected on some interfaces.
Solution:
The fault is very likely caused if the multiplex unit configurations on the SDH
transmission device mismatch the E1 channel numbers on the CPOS interface on
your device. This can result in timeslot inconsistency at the two ends of
transmission and as such, PPP negotiation failures and LCP anomalies.
Besides, if an idle timeslot on a loopback serial interface on the transmission
device is used in transmission, the information that loopback is detected is
displayed. Use the debugging ppp lcp error command to check loopback
information.
Follow these steps to solve the problem:
Use the display controller cpos e1 command to view the multiplexing paths
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of the E1 channels or calculate the multiplexing path as shown in section
"Calculating E1/T1 Channel Sequence Numbers" on page
Check the configurations on the transmission devices against the calculating
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result in the last step to make sure the same E1 multiplexing path is configured.
Troubleshooting CPOS Interfaces
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