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.