RFC 3623 describes a way for OSPF control functions to restart without disrupting traffic forwarding.
Without graceful restart, adjacent routers will assume that information previously received from the restarting router is stale and will not be used to forward traffic to that router. However, in many cases, two conditions exist that allow the router restarting OSPF to continue to forward traffic correctly. The first condition is that forwarding can continue while the control function is restarted. Most modern router system designs separate the forwarding function from the control function so that traffic can still be forwarded independent of the state of the OSPF function. Routes learned through OSPF remain in the routing table and packets continue to be forwarded. The second condition required for graceful restart is that the network remain stable during the restart period. If the network topology is not changing, the current routing table remains correct. Often, networks can remain stable during the time for restarting OSPF.