How to troubleshoot the fault of LDP session flapping due to the expiration of Hello Hold timer

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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.

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

How are LDP Hello messages identified
The destination address of Hello messages in a local session is 224.0.0.2 and the source address is the IP address of the LDP-enabled interface. The destination address of Hello messages in a remote session is the LSR ID of the peer end and the source address is the LSR ID of the local end. Hello packets are UDP packets and are sent through LDP-dedicated port 646. Hello messages are forwarded based on IP routing.

How to set the durations of the Hold timer and Keepalive timer of the BGP peer
You can run the peer timer command to set the durations of the Hold timer and Keepalive timer. This command specifies the timeout period of the BGP connection and the interval to send the Keepalive message. A longer timeout period can relieve the impact of link flapping, whereas a shorter timeout period makes the timer rapidly perceive link changes. After connections are established between two peers, the durations of the two timers are negotiated by the two peers and the shorter durations are adopted.

What are the functions of LDP Hello messages
An LSR maintains the adjacency relationship with another LSR through received Hello messages. The LSR maintains the Hello Hold timer of the Hello adjacency. Upon receiving a Hello message matching the Hello adjacency, the Hello Hold timer is reset. If the Hello Hold timer expires, the LSR deletes the Hello adjacency. If the LDP session has only one Hello adjacency, the LSR deletes the LDP session.

Why is the LDP GR duration in the log longer than 10 minutes, which is the interval of the MPLS Forwarding State Holding timer
After the GR restarter performs an active/standby switchover, LDP GR starts only after route GR is complete. The MPLS Forwarding State Holding timer is started immediately after the active/standby switchover but restarted when route GR is complete. In the log, the GR begin time is the time when the MPLS Forwarding State Holding timer is first started; the GR end time is the time when the MPLS Forwarding State Holding timer expires. Therefore, the LDP GR duration recorded in the log is the sum of the route GR duration and MPLS Forwarding State Holding timer interval (10 minutes).

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