The establishment of LDP LSPs is accomplished by PE and P devices through the use of the Label Distribution Protocol. LDP is responsible for assigning tunnel labels to an LDP LSP and for handling error conditions.
An LDP LSP is PE to PE.
PE and P devices exchange LDP Link Hello messages to locate and establish LDP sessions with their directly connected neighbors. Three such sessions, or adjacencies, are shown in Non-targeted LDP adjacencies.
Peer LDP speakers establish a TCP-based communications session with each other. Once the session is established, tunnel label data can be exchanged to construct LDP LSPs.
The MPLS Topology Server discovers the LDP speakers and LDP sessions that are associated with LDP LSPs. The MPLS Topology Server also discovers the LDP speakers and LDP sessions that are associated with L2VPNs, as described in “More about targeted LDP adjacencies and LDP adjacency discovery” on page 93.