Loop and link layer protocol down state detected on serial interfaces created for E1 channels on the CPOS interface
Symptom
The HPE router is connected to another vendor's router through E1 channels on CPOS interfaces across an SDH network. PPP is used on the serial interface created for the E1 channel set.
The output from the display interface serial command shows the following errors:
The physical state of the serial interface is up, but the link protocol is down.
The serial interface is in a looped condition.
Solution
The symptom might occur when the router and its directly connected SDH device have different multiplex paths for the E1 channels. Multiplex path inconsistency can cause PPP negotiation failure because the SDH device transmits signals from the router in incorrect timeslots to the remote end. If the SDH device incorrectly maps a signal to an idle timeslot in a looped condition, the router can detect a loop on the serial interface.
To resolve the problem:
Identify the multiplex path for the E1 channels on the router.
<HPE> display controller cpos e1
Verify that the router and its directly connected SDH device have the same multiplex path for E1 channels. (Details not shown.)
Debug the loop condition.
<HPE> debugging ppp lcp error
If the problem persists, contact Hewlett Packard Enterprise Support.