Label withdrawal delay behaves differently when LDP graceful restart is executed by the helper node or the restarting node.
When graceful restart is executed by the helper node, graceful restart helper procedures are initiated when a session to a peer goes down. During the session down processing, the label mappings exchanged with a peer are preserved while the peer is reconnecting. If the peer session is reconnected within a configurable time limit then the label mappings previously exchanged with the peer are refreshed with new label mappings. Any mappings that are not refreshed are released.
When both label withdrawal delay and GR are enabled, the label withdrawal delay timer is not initiated when the session goes down because the session is considered to be in a special restarting state and not actually down. If the session is not re-established within the reconnect time for GR then the session is considered to be down and the label withdrawal delay timer may be started for any FECs which meet the criteria for label withdrawal delay. If the session is re-established within the reconnect time for GR then the label withdrawal delay timer is not started.
Individual FECs may experience some transition as the label mappings from the peer are refreshed. For example, if a label mapping is not refreshed during the restart window, then that label mapping will be removed. This will affect the FEC, especially if it was associated with an installed downstream mapping. If an alternate route exists, the FEC will re-converge on the alternate route much earlier and again the label withdrawal delay timer is not started for the FEC
When graceful restart is executed by the restarting node, the GR restarting procedures are executed by a node which is starting the LDP process and has retained the forwarding state for LDP connections from an earlier instance of LDP. During the forwarding state hold period, connections in the forwarding state are marked stale until they are refreshed by an additional label mapping. Stale connections are removed after this period.
This scenario does not affect label withdrawal delay since it involves a complete restart of the LDP control plane. In other words, there are no FECs or sessions to apply the label withdrawal delay timer to at restart. Other procedures for label withdrawal delay as described previously may occur in the same way during the forwarding state hold period.