12.5.3 Restart function

The restart function of OSPF operates at the following timing.

When switching of the master switch occurs in the stack configuration.
When the unicast routing program restarts.

<Structure of this section>

(1) OpaqueLSA

Learn and advertise Type9's Opaque LSA at the beginning and end of a graceful restart. However, Opaque LSA of Type9 does not support features other than grace-LSA used for graceful restart of OSPF.

(2) Graceful restart procedure

The following figures and tables show the graceful restart procedure for OSPF.

Figure 12-7: OSPF graceful restart procedure

Table 12-5 OSPF graceful restart procedure
No.	Item	Trigger	Process Description
1	Starting graceful restart	When the master switch is switched during stack configuration.	Starts a graceful restart. As with normal connectivity, interfaces exchange OSPF packets.
1	Starting graceful restart	When the unicast routing program restarts.
2	Route calculation	When reconnection is completed for all OSPF interfaces in the domain, and all LSA are learned from neighboring routers.	Route calculations are performed for each domain, and the routing table is updated. If there are multiple domains, the route calculation is performed from time to time from the domain where the connection is terminated. When route computation is completed for all domains, OSPF route learning is completed.
2	Route calculation	A graceful restart has failed on a single interface.	Route calculations are based on the connection status of each interface in the same domain at that time.
3	Start of advertising	When route learning for OSPF is complete and route learning for other routing protocols is complete.	Advertisement of external AS routes starts. After the advertisement is completed, OSPF returns to normal operation.
3	Start of advertising	A graceful restart of OSPF failed.

(3) When graceful restart fails

The graceful restart of OSPF fails in the following cases:

When LSA learning cannot be completed even after the restart time (specified in graceful-restart restart-time of OSPF configuration command) elapses after the graceful restart has been notified to the helper router
When the reconnected interface goes down
If LSA is changed on OSPF
When another router on OSPF domain performs a graceful restart
When route learning for all protocols is not completed even after the route retention period (specified by the configuration command routing options graceful-restart time-limit) elapses after graceful restart starts
When the restart function is deleted by changing the configuration command graceful-restart mode

(4) Notes

Even if graceful restart is started as a restart router, if some helper routers do not start helper operation or stop in the middle, graceful restart is stopped on all interfaces in the same domain.
Set the restart time of OSPF to exceed the switching time of the master switch + LSA learning time. This is because in order for OSPF to learn LSA, the master switch must be switched and Up/Down of IP interfaces must be visible. After a graceful restart starts, if LSA has not been learned after the restart period has elapsed, OSPF graceful restart fails.
Set the retention time for routing entries when the Switch restarts gracefully longer than OSPF restart time. If the routing entry retention time is shorter than OSPF restart time, routing entries prior to graceful restart might be deleted prior to route learning.
If the internal peer of BGP4 is using graceful restart, set the restart time of the internal peer longer than the restart time of OSPF. If the internal peer restart time is shorter, the internal peer might fail to gracefully restart because the internal peer cannot be connected before OSPF has completed route learning.