How are LDP Hello messages identified

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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.

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 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 check the statistics on received and sent Hello messages
Run the display mpls ldp interface command to check the statistics on received and sent Hello messages on an interface. The command output is displayed as follows: display mpls ldp interface LDP Interface Information in Public Network Codes:LAM(Label Advertisement Mode), IFName(Interface name) A '*' before an interface means the entity is being deleted. ------------------------------------------------------------------------------ IFName Status LAM TransportAddress HelloSent/Rcv ------------------------------------------------------------------------------ Vlanif10 Active DU 1.1.1.1 29574/29539 ------------------------------------------------------------------------------ The value of the field HelloSent/Rcv indicates the statistics on received and sent Hello messages

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 are statistics on received and sent Hello messages both 0
Run the display mpls ldp interface command to check the interface status. The command output is displayed as follows: display mpls ldp interface LDP Interface Information in Public Network Codes:LAM(Label Advertisement Mode), IFName(Interface name) A '*' before an interface means the entity is being deleted. ------------------------------------------------------------------------------ IFName Status LAM TransportAddress HelloSent/Rcv ------------------------------------------------------------------------------ Vlanif10 Active DU 1.1.1.1 29574/29539 ------------------------------------------------------------------------------ In normal cases, the interface status is Active. If the interface status is Inactive, the interface cannot receive or send Hello messages. In most cases, the interface status is Inactive because the interface is shut down or no IP address is configured for the interface.

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