OSPF Graceful Restart directs OSPF neighbors
to continue using routes through a firewall during a short transition
when it is out of service. This behavior increases network stability
by reducing the frequency of routing table reconfiguration and the
related route flapping that can occur during short periodic down
times.
For a Palo Alto Networks firewall, OSPF Graceful Restart
involves the following operations:
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.