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

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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.

Other related questions:
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.

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.

On S series switches, what are the methods of defining the values of Hold and Keepalive timers on BGP peers
For S series switches that support BGP, the value of Hold and Keepalive timers can be set with the peer timer command. This command specifies the hold time when the BGP connection is interrupted and the interval for sending Keepalive messages. Large values of the two timers reduce the impact of link flapping, and small values enable the device to detect link changes. After the peer relationship is set up, the values of the two timers are negotiated by both peers, and the smaller ones are used.

What are the functions of LDP KeepAlive messages
An LSR maintains the LDP session with another LSR based on the received KeepAlive messages. The LSR maintains the KeepAlive Hold timer of the LDP session. Upon receiving a KeepAlive message matching the LDP session, the KeepAlive Hold timer is reset. If the KeepAlive Hold timer expires, 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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