LDP uses Hello messages to determine neighbors and maintain up status with neighbors.  LDP multicasts to using UDP port 646 for LDP.  Older TDP uses UDP port 711.

Hellos contain a 32 bit dot decimal number and 2 byte label space number referred to as a LSR LDP ID (LID)

LSRs can optionally send a transport address in the Hello messages to allow neighbors to send any TCP communications to work.  If a transport address is not used, hosts will use the IP address in the first 4 bytes of the LDP ID for TCP connections

Once neighbors are found they establish a TCP connection using TCP 646 (or older TCP 711 for TDP).  Once a connection is up each router advertises its list of local labels and prefix bindings.



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