Firewall as a restarting device—If the firewall will be down for a short period of time or is unavailable for short intervals, it sends Grace LSAs to its OSPF neighbors. The neighbors must be configured to run in Graceful Restart helper mode. In helper mode, the neighbor receives Grace LSAs informing it that the firewall will perform a graceful restart within a specified period of time defined as the Grace Period. During the grace period, the neighbor continues to forward routes through the firewall and to send LSAs that announce routes through the firewall. If the firewall resumes operation before expiration of the grace period, traffic forwarding will continue as before without network disruption. If the firewall does not resume operation after the grace period has expired, the neighbors will exit helper mode and resume normal operation, which will involve reconfiguring the routing table to bypass the firewall.

Firewall as a Graceful Restart Helper—If neighboring routers may be down for short periods of time, the firewall can be configured to operate in Graceful Restart helper mode, in which case the firewall employs a Max Neighbor Restart Time. When the firewall receives the Grace LSAs from its OSPF neighbor, it continues to route traffic to the neighbor and advertise routes through the neighbor until either the grace period or max neighbor restart time expires. If neither expires before the neighbor returns to service, traffic forwarding continues as before without network disruption. If either period expires before the neighbor returns to service, the firewall exits helper mode and resumes normal operation, which involves reconfiguring the routing table to bypass the neighbor.