LDP Longest Match for Labels

LDP is often used to establish MPLS label-switched paths (LSPs) throughout a complete network domain using an interior gateway protocol (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.

On a Junos device, 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 (for example, /32) to be distributed through IGP across the entire network domain. This defeats the purpose of IP hierarchical design or default routing in access devices. You can use this feature to enable the longest match, allowing LDP to set up LSPs toward unique endpoint IP addresses (for example, /32) based on the routes aggregated or summarized (for example, /24, or even /0 for the default route) across OSPF areas or IS-IS levels in the inter-domain.