3Com MSR 50 Series Configuration Manual page 1457

Check whether the PEs are configured with the Remote argument and whether
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the peer addresses are correctly configured.
Symptom 2:
Serial 1/0 and Serial 1/1 are configured with different encapsulation types: one is
HDLC and the other PPP. They create an LDP connection respectively, with the
same VC ID of 1. If you change the encapsulation type of Serial 1/1 to HDLC, the
expected LDP connection cannot be established.
Analysis:
When you change the encapsulation type of Serial 1/1 to HDLC, another LDP
connection is established, with the same encapsulation type of HDLC and the
same VC ID of 1. To avoid VC ID conflict, the system removes the new LDP
connection automatically.
Solution:
Configure different VC IDs for the two interfaces.
Troubleshooting MPLS L2VPN 1457