LDP Neighbor Discovery
LDP includes a neighbor discovery protocol that runs over UDP.
Using the basic discovery mechanism, each LSR periodically multicasts a hello message to a well-known UDP port to which all LSRs listen. These hello messages are transmitted to the all routers on this subnet multicast group. When a neighbor is discovered, a hello-adjacency is formed and the LSR with the numerically greater IP address is denoted as the active LSR.
Hello messages must continue to be received periodically for the hello-adjacency to be maintained. The hold time that specifies the duration for which a hello message remains valid can be negotiated by the peer LSRs as part of the HELLO exchange. During the HELLO exchange, each LSR proposes a value and the lower of the two is used as the hold time.
Targeted LDP hello-adjacencies between potentially non-directly connected LSRs are supported using an extended discovery mechanism. In this case, targeted hello messages are periodically sent to a specific IP address.
After the hello-adjacency is formed, the active LSR initiates establishment of a TCP connection to the peer LSR. At this point, an LDP session is initiated over the TCP connection. The LDP session consists of an exchange of LDP messages that are used to set up, maintain, and release the session.