What are the functions of LDP KeepAlive messages

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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.

Other related questions:
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.

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 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.

What functions do GR timers provide in the MPLS LDP view
After GR Restarter performs an active/standby switchover, the MPLS Forwarding State Holding Timer is started, which is the Neighbor-liveness timer in the MPLS LDP view. Before the Neighbor-liveness timer expires, all forwarding entries before the active/standby switchover are retained. After the Neighbor-liveness timer expires, the GR Helper deletes the forwarding entries that are not restored. When detecting that the GR Restarter performs an active/standby switchover or the LDP protocol restarts, the GR Helper starts the GR Reconnect timer. Before the GR Reconnect timer expires, the GR Helper retains all forwarding entries related to the GR Restarter. After the GR Reconnect timer expires, the GR Helper deletes the forwarding entries that are not restored. If a session is re-established between the GR Restarter and Helper before the GR Reconnect timer expires, the GR Helper deletes the GR Reconnect timer and starts the GR Recovery timer. Before the GR Recovery timer expires, the GR Helper helps the GR Restarter restore forwarding entries. After the GR Recovery timer times out, the GR Helper deletes all the GR Restarter-related forwarding entries that are not restored.

What are the functions of the DHCP Request message
The DHCP Request message provides the following functions: --Responds to the Offer message sent by the server. The DHCP Request message notifies the selected server during address application as it contains the server identifier option. --Checks the assigned network address. --Applies for the valid period of the address. --Extends the existing lease and prolongs the lease period.

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