LDP Synchronization

Synchronization between LDP and the IS-IS interior gateway protocol (IGP) ensures that LDP is fully established before the IGP path is used for forwarding traffic. LDP is often used to establish MPLS label-switched paths (LSPs) throughout a network domain that uses IS-IS. In such a network, all links in the domain have IS-IS adjacencies as well as LDP adjacencies. LDP establishes the LSPs on the shortest path to a destination as determined by IP forwarding. If IS-IS and LDP are not synchronized, meaning IS-IS adjacency is fully operational while the corresponding LDP adjacency is not fully operational, packet loss can occur. This issue is significant for applications such as a core network that does not employ BGP. Another example is an MPLS VPN where each provider edge (PE) device depends on the availability of a complete MPLS forwarding path to the other PE devices for each VPN that it serves. This means that along the shortest path between the PE devices, each link must have an operational hello adjacency and an operational LDP session, and MPLS label bindings must have been exchanged over each session. LDP establishes MPLS LSPs along the shortest path to the destination as determined by IP forwarding. In a Layer 2 VPN or Layer 3 VPN scenario, if the LSP is not yet formed between the PE devices, services depending on MPLS forwarding fail. When LDP has not completed exchanging label bindings with an IGP next hop, traffic is discarded if the head end of the LSP forwards traffic because the LSP is assumed to be in place.