LDP Graceful Restart
LDP graceful restart enables a router or switch whose LDP control plane is undergoing a restart to continue to forward traffic while recovering its state from neighboring routers or switches. It also enables a router or switch on which helper mode is enabled to assist a neighboring router or switch that is attempting to restart LDP.
During session initialization, a router or switch advertises its ability to perform LDP graceful restart or to take advantage of a neighbor performing LDP graceful restart by sending the graceful restart TLV. This TLV contains two fields relevant to LDP graceful restart: the reconnect time and the recovery time. The values of the reconnect and recovery times indicate the graceful restart capabilities supported by the router or switch.
When a router or switch discovers that a neighboring router or switch is restarting, it waits until the end of the recovery time before attempting to reconnect. The recovery time is the length of time a router or switch waits for LDP to restart gracefully. The recovery time period begins when an initialization message is sent or received. This time period is also typically the length of time that a neighboring router or switch maintains its information about the restarting router or switch, allowing it to continue to forward traffic.
You can configure LDP graceful restart in both the master instance for the LDP protocol and for a specific routing instance. You can disable graceful restart at the global level for all protocols, at the protocol level for LDP only, and on a specific routing instance. LDP graceful restart is disabled by default, because at the global level, graceful restart is disabled by default. However, helper mode (the ability to assist a neighboring router or switch attempting a graceful restart) is enabled by default.
The following are some of the behaviors associated with LDP graceful restart:
- Outgoing labels are not maintained in restarts. New outgoing labels are allocated.
- When a router or switch is restarting, no label-map messages are sent to neighbors that support graceful restart until the restarting router or switch has stabilized (label-map messages are immediately sent to neighbors that do not support graceful restart). However, all other messages (keepalive, address-message, notification, and release) are sent as usual. Distributing these other messages prevents the router from distributing incomplete information.
- Helper mode and graceful restart are independent. You can disable graceful restart in the configuration, but still allow the router or switch to cooperate with a neighbor attempting to restart gracefully.
