How to check the LDP TCP connections and determine whether TCP connections are normal

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The port number for LDP-dedicated TCP connections is 646. Run the display tcp status command. All connections with port number of 646 are LDP TCP connections. In most cases, the server has a Listening port and one or more Established connections. The local port number is 646, and the remote port number is a randomly allocated five-digit numeral. The command output is displayed as follows:
display tcp status
TCPCB Tid/Soid Local Add:port Foreign Add:port VPNID State
0d3bda5c 32 /1 0.0.0.0:23 0.0.0.0:0 14849 Listening
0b6d91cc 23 /2 2.2.2.2:646 1.1.1.1:53585 0 Established

Other related questions:
How do I check the TCP connection established using LDP
LDP uses TCP port 646. Run the display tcp status command. If port 646 is used, TCP connections are established using LDP.

How are the server and client determined for establishing a TCP connection
Check the transport addresses used to establish a TCP connection. The larger address is the active end that initiates the TCP connection. It is the client of the TCP connection. The smaller address is the server of the TCP connection.

How to determine the interface address used to establish a TCP connection
By default, the LSR ID is used to establish a TCP connection. If the mpls ldp transport-address command is configured in the interface view, the address of the specified interface is used to establish a TCP connection.

How to check the transport addresses used to establish a TCP connection
To check the transport addresses used to establish a TCP connection, run the display mpls ldp session verbose command.

Why does the TCP connection fail even if the server and client both initiate a TCP connection
LDP sessions are established based on TCP connections using MD5 authentication. The TCP connection cannot be established, if one end is configured with MD5 authentication while the other end is not, or the MD5 passwords on the two ends are different. If MD5 cipher text authentication is proprietary on other vendors' devices, Huawei devices cannot set up a TCP connection with these devices.

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