Label Distribution Protocol

LDP is used to build and maintain label-switched path (LSP) databases that are used to forward traffic through MPLS networks.

During the discovery phase hello packets are sent on UDP port 646 to the 'all routers on this subnet' group multicast address (224.0.0.2).

LDP relies on the underlying routing information provided by an IGP in order to forward label packets.

Use of authenticated LDP-over-TCP helps provide strong integrity protection against misconfigured would-be LDP peers.

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.