Introduction

LDP is only used for label distribution and not for routing-decision functions. IGP advertised IPv4 prefixes, while LDP creates labels for them and distributes to neighbors.

MPLS LDP IGP Interaction

• For each IGP prefix, LDP binds a local label to it. It ignores BGP routes.
• IPv4 label bindings get advertised to other LDP neighbors.
• LDP does not perform any routing-related decisions, it relies on IGP for that. LDP primarily assigns local bindings and advertises these to the neighbors.
• Each LSR will pick the label that corresponds to the Shortest IGP path. If multiple equal paths exist, packets are load balanced. See MPLS Forwarding Load Balancing for details.
• LSP's established using LDP, always follow the shortest IGP path and changes upon IGP network convergence.
• If the synchronization is lost between IGP and LDP there is a potential for packet loss. See MPLS-LDP-IGP-Synchronization for details.

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