Understanding the LDP Signaling 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.

When you configure LDP on a label-switching router (LSR), the router begins sending LDP discovery messages out all LDP-enabled interfaces. When an adjacent LSR receives LDP discovery messages, it establishes an underlying TCP session. An LDP session is then created on top of the TCP session. The TCP three-way handshake ensures that the LDP session has bidirectional connectivity. After they establish the LDP session, the LDP neighbors maintain, and terminate, the session by exchanging messages. LDP advertisement messages allow LSRs to exchange label information to determine the next hops within a particular LSP. Any topology changes, such as a router failure, generate LDP notifications that can terminate the LDP session or generate additional LDP advertisements to propagate an LSP change.

Starting in Junos OS Release 20.3R1, support for MPLS to provide LDP signaling protocol configuration with the control plane functionality.