What are the reasons for failure in establishing MPLS LDP sessions

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An MPLS LDP session fails to be established due to the following possible causes:
The route to the peer is not generated, or MPLS LDP is disabled on the outbound interface of the route to the peer.
LDP protocol packets have been limited by CPCAR on the device, and LDP protocol packets cannot be properly processed.

Other related questions:
Why is no MPLS LSP generated after an MPLS LDP session is established
After a remote MPLS LDP session is established, the corresponding MPLS LSP cannot be generated if MPLS LDP is disabled on some interfaces. When MPLS LDP is disabled on an interface, the remote LDP session can be established. The LSP labels, however, are allocated hop by hop, which requires that MPLS LDP be enabled on all interfaces along the LSP.

What information needs to be collected if an MPLS LDP session fails to be established on an S series switch
For S series switches (except the S1700), you need to collect the following information if an MPLS LDP session fails to be established: 1. Detailed information about the session status, which is displayed using the display mpls ldp session verbose command. 2. LDP status (local or remote), which is displayed using the display mpls ldp peer verbose command. 3. Information about sent and received LDP packets on the interface, which is displayed using the display mpls ldp interface [verbose] command. If MPLS LDP is disabled on the interface, no command output is displayed. 4. Information about sent and received LDP packets after the remote session is established, which is displayed using the display mpls ldp remote-peer peer-name command. 5. Information about whether the route to the peer exists, which is displayed using the display ip routing-table x.x.x.x verbose or display fib x.x.x.x verbose command. 6. Information about whether the TCP connection is in Established state, which is displayed using the display tcp status command. 7. Reason why the LDP session is Down, which is displayed using the display mpls ldp event session-down command.

How do I check the TCP connection established using LDP
LDP uses TCP port 646. Run the display tcp status command. If port 646 is used, TCP connections are established using LDP.

Does an AR support MPLS LDP
Support for MPLS LDP: - The AR120&AR150&AR160&AR200&AR500&AR510 do not support MPLS LDP. - The AR120-S&AR150-S&AR160-S&AR200-S do not support MPLS LDP. Other -S models support MPLS LDP starting from V200R005C10. - The AR530 and AR550 do not support MPLS LDP. - Other models support MPLS LDP.

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