Label Distribution Protocol

Label Distribution Protocol

Label Distribution Protocol (LDP) is a protocol in which two Label Switch Routers (LSR) exchange label mapping information. The two LSRs are called LDP peers and the exchange of information is bi-directional.LDP is used to build and maintain LSR databases that are used to forward traffic through MPLS networks.

LDP can be used to distribute the inner label (VC label) and outer label in MPLS. For inner label distribution targeted LDP (tLDP) is used. tLDP runs on TCP port 646 as a targeted session.


The Label Distribution Protocol (LDP) is a protocol defined by the IETF for the purpose of distributing labels in an MPLS environment. LDP relies on the underlying routing information provided by an IGP in order to forward label packets. The router Forwarding Information Base, or FIB, is responsible for determining the hop-by-hop path through the network. Unlike traffic engineered paths, which use constraints and explicit routes to establish end-to-end LSPs, LDP is used only for signaling best-effort LSPs.


Targeted LDP sessions are LDP sessions between non-directly connected peers. When an SDP is configured, automatic ingress and egress labeling (targeted LDP) is enabled by default and ingress and egress “service” labels are signaled over a TLDP connection. If signaling is turned off on an SDP, ingress and egress “service” labels must be manually configured when the SDP is bound to a service.


This method determines a path through the network based on the interior gateway protocol's view of the network. If no constraints are applied to the LSP then the routers simply send the request for a path to the active next hop for that destination, without explicit routing. The IGP at each router is free to select active next hops based on the link state database.

See also

* Resource Reservation Protocol
* Constraint-based Routing Label Distribution Protocol - CR-LDP

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