Which requirements are there for successfully establishing an LDP LSP


The requirements for successfully establishing an LDP LSP include:
Run the display mpls ldp session command to check whether an LDP session is correctly set up.
Run the display ip routing-table command to check whether reachable routes are generated.
Run the display mpls ldp peer command to check whether the route outbound interface is the same as the discovery source of the LDP peer.
Check whether the configuration meets the triggering condition for establishing LSPs. You can check the triggering condition for establishing LSPs in the MPLS view. The default configuration is lsp-trigger host.
If the LSP fails to be established although the preceding requirements are met, run the debugging mpls ldp fec command to check the process of establishing this LSP.

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 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 LDP loop detection not required on the AR
LDP is generated based on IP routes. A routing protocol such as IS-IS or OSPF can avoid routing loop generation, so LDP loop detection is not required any more.

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.

Why cannot an LSP session become Up after a TCP connection is established
After a TCP connection is established, the two ends of the TCP connection negotiate parameters such as label distribution mode (DU or DoD). If negotiation fails, the LSP session cannot become Up.

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