Why is an LDP session suddenly Down

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An LDP session is sensitive to network link status and how busy the system is. If packet transmission fails or the CPU usage is high, LDP session flapping may occur. If an LDP session is disconnected, the system records it in the log. Common causes are timing-out of the Hello Hold timer and KeepAlive Hold timer.

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.

Why is LDP loop detection unnecessary
LSPs are generated strictly based on IP routing. Routing protocols such as OSPF and IS-IS have a mechanism to ensure loop-free routes. Therefore, LDP loop detection is unnecessary.

How to troubleshoot the fault of LDP session flapping due to the expiration of Hello Hold timer
If the Hello Hold timer expires, run the display mpls ldp interface command to check whether Hello messages are properly sent on the two ends of the LDP session. If not, the system is busy or faulty. If Hello messages are properly sent but the peer end does not receive them, ping the address of the peer end to check the forwarding status.

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