Label Distribution Protocol (LDP)

Label Distribution Protocol (LDP) is a signaling protocol that runs on a device configured for MPLS support. The successful configuration of both MPLS and LDP initiates the exchange of TCP packets across the LDP interfaces. The packets establish TCP-based LDP sessions for the exchange of MPLS information within the network. Enabling both MPLS and LDP on the appropriate interfaces is sufficient to establish LSPs.

LDP is a simple, fast-acting signaling protocol that automatically establishes LSP adjacencies within an MPLS network. Routers then share LSP updates such as hello packets and LSP advertisements across the adjacencies. Because LDP runs on top of an IGP such as IS-IS or OSPF, you must configure LDP and the IGP on the same set of interfaces. After both are configured, LDP begins transmitting and receiving LDP messages through all LDP-enabled interfaces. Because of LDP's simplicity, it cannot perform the true traffic engineering which RSVP can perform. LDP does not support bandwidth reservation or traffic constraints.