Longest Match Support for LDP Overview

LDP is often used to establish MPLS label-switched paths (LSPs) throughout a complete network domain using an IGP such as OSPF or IS-IS. In such a network, all links in the domain have IGP adjacencies as well as LDP adjacencies. LDP establishes the LSPs on the shortest path to a destination as determined by IGP. In Junos OS, the LDP implementation does an exact match lookup on the IP address of the forwarding equivalence class (FEC) in the routing information base (RIB) or IGP routes for label mapping. This exact mapping requires MPLS end-to-end LDP endpoint IP addresses to be configured in all the label edge routers (LERs). This defeats the purpose of IP hierarchical design or default routing in access devices. Configuring longest-match allows LDP to set up LSP based on the routes aggregated or summarized across OSPF areas or IS-IS levels in the inter-domain.