LDP Graceful Restart

LDP graceful restart (GR) minimizes the negative effects on MPLS traffic caused by control-plane restarts in Label Switching Routers (LSR), especially by the restart of the Label Distribution Protocol (LDP).

LDP graceful restart enables a router whose LDP control plane is undergoing a restart to continue forwarding traffic while recovering its state from neighboring routers. This requires a restarting LDP router that retains established LSP labels. In helper mode, the router maintains label bindings as stale and reprocesses them, after the router undergoing graceful restart reestablishes its LDP session.

The MPLS forwarding state, which is the minimum state required to avoid any disturbance to LSPs traversing a restarting LSR, is preserved during the restart. This mechanism does not require any of the LDP-related states to be preserved across the restart. This means that when LDP restarts, there are minimal or no changes made to the forwarding table entries, and MPLS forwarding continues uninterrupted. This supports Graceful Restart in restarting routers as well as in neighbor routers.

Topology

Figure 81. LDP Graceful Restart