Graceful Restart

OSPF graceful restart, sometimes referred to as non-stop forwarding, provides for an OSPF router to remain on the forwarding path during a restart of its OSPF software. Graceful-restart has three elements to its configuration: enabling, helper router, and restart interval.

Enabling graceful restart instructs the firmware to perform a graceful restart, rather than a standard OSPF restart. Restart is only initiated by a fail-over. Grace LSAs are sent when OSPF is restarted on another module. Whether the failover is intentional or not, the failed router protocol is restarted on another module, and upon startup, OSPF sends grace LSAs out to its neighbors using existing link aggregation groups. Use the graceful-restart enable command to enable the graceful restart ability on this router.

The helper relationship with the restarting router is on a per network segment basis. The helper monitors the network for topology changes. If no changes occur, the helper router continues to advertise its LSAs as though no restart was occurring. If the restarting router was the designated router, the helper continues to treat it as such. If a topology change does occur, graceful restart is terminated on the restarting router and a standard restart occurs. Helper mode can be disabled on a restarting router neighbor using the ip ospf helper-disable command in interface command mode. If the restarting router receives an LSA indicating a disabled helper, the graceful restart terminates and a standard restart occurs.

A restart interval provides for a maximum time in seconds after which the graceful restart will terminate should it not complete or terminate for other reasons within the interval. Use the graceful-restart restart-interval command to change the restart interval setting.

View the router OSPF section of the show running-config display to verify any non-default graceful restart settings.