ldp

LDP, or Label Distribution Protocol, is a control-plane protocol used in MPLS networks to exchange label bindings between label switch routers (LSRs). LDP enables the establishment of label-switched paths by distributing labels for Forwarding Equivalence Classes (FECs), allowing MPLS routers to forward packets based on short labels rather than IP headers.

Operation in typical deployments relies on TCP sessions. LDP peers discover each other by sending Hello messages on their interfaces and establish a TCP connection (default port 646). Once a session is up, routers exchange label mappings, requests, and withdrawals to bind local and remote FECs to labels. Label mappings may be advertised for IPv4, IPv6, or other FEC types. LDP provides mechanisms for both simple, directly connected adjacencies and more complex topologies through targeted sessions to non-direct neighbors.

Variants and topology control: Standard LDP is used between directly connected LSRs to propagate label bindings across an MPLS domain. Targeted LDP extends label distribution to non-adjacent peers by initiating TCP sessions to specific neighbor addresses, enabling label distribution across hub-and-spoke or multi-hop setups without requiring direct adjacency on every link.

Relation to MPLS and traffic engineering: LDP is a common mechanism to supply label bindings that enable MPLS forwarding. In networks that require traffic engineering, LDP can operate alongside or be complemented by other label-distribution methods, such as RSVP-TE or MP-BGP-based approaches, depending on the desired routing and TE architecture.

Security and reliability: LDP relies on TCP for reliable delivery and can be protected by TCP authentication where supported. Sessions are kept alive with keep-alive messages and are reestablished if a link or neighbor changes.