What information needs to be collected if an MPLS LDP session fails to be established on an S series switch

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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.

Other related questions:
What are the reasons for failure in establishing MPLS LDP sessions
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.

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.

Collect diagnostic information on s series switch
For S series switches except S1700 switches, you can run the display diagnostic-information command to collect and display current diagnosis information or store the information in a specified file.

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.

What is MPLS VPN on S series switches
MPLS VPN on S series switches (except S1700 switches): The BGP/MPLS IP VPN model consists of the following parts: Customer Edge (CE): an edge device on a customer network. A CE provides interfaces that are directly connected to an SP network. A CE can be a router, switch, or host. In most cases, CEs cannot detect VPNs and do not need to support MPLS. Provider Edge (PE): an edge device on an SP network, which is directly connected to a CE. On the MPLS network, all handlings related to the VPN are performed on a PE, so the requirement on the PE performance is high. Provider (P): a backbone device on an SP network, which is not directly connected to CEs. A P device only needs to provide basic MPLS forwarding capabilities, without maintaining VPN information. PEs and Ps are managed by SPs. CEs are managed by users unless the users entrust CE management to SPs. A PE can be connected to multiple CEs. A CE can be connected to multiple PEs of the same SP or of different SPs.

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