Protocol Restoration; Graceful Restart Requirements And Restrictions - Cisco ASR 9000 Series Routing Configuration Manual

Graceful Shutdown for OSPFv3
The protocol shutdown mode can be invoked either manually through the protocol shutdown command that
disables the protocol instance or when the OSPFv3 process runs out of memory. These events occur when
protocol shut down is performed:
• The local Router LSA and all local Link LSAs are flushed. All other LSAs are eventually aged out by
other OSPFv3 routers in the domain.
• OSPFv3 neighbors not yet in Full state with the local router are brought down with the Kill_Nbr event.
• After a three second delay, empty Hello packets are immediately sent to each neighbor that has an active
adjacency.
After a Dead Hello interval delay (4 X Hello Interval), the following events are then performed:
• The LSA database from that OSPFv3 instance is cleared.
• All routes from RIB that were installed by OSPFv3 are purged.
The router will not respond to any OSPF control packets it receives from neighbors while in protocol shutdown
state.

Protocol Restoration

The method of restoring the protocol is dependent on the trigger that originally invoked the shut down. If the
OSPFv3 was shut down using the protocol shutdown command, then use the no protocol shutdown command
to restore OSPFv3 back to normal operation. If the OSPFv3 was shutdown due to a Critical Memory message
from the sysmon, then a Normal Memory message from sysmon, which indicates that sufficient memory has
been restored to the processor, restores the OSPFv3 protocol to resume normal operation. When OSPFv3 is
shutdown due to the Critical Memory trigger, it must be manually restarted when normal memory levels are
restored on the route processor. It will not automatically restore itself.
These events occur when the OSPFv3 is restored:
1 All OSPFv3 interfaces are brought back up using the Hello packets and database exchange.
2 The local router and link LSAs are rebuilt and advertised.
3 The router replies normally to all OSPFv3 control messages received from neighbors.
4 Routes learned from other OSPFv3 routers are installed in RIB.

Graceful Restart Requirements and Restrictions

The requirements for supporting the Graceful Restart feature include:
• Cooperation of a router's neighbors during a graceful restart. In relation to the router on which OSPFv3
is restarting, each router is called a helper.
• All neighbors of the router that does a graceful restart must be capable of doing a graceful restart.
• A graceful restart does not occur upon the first-time startup of a router.
• OSPFv3 neighbor information and database information are not check-pointed.
• An OSPFv3 process rebuilds adjacencies after it restarts.
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◦ An empty Hello packet is sent periodically until the dead_interval has elapsed.
◦ When the dead_interval elapses, Hello packets are no longer sent.
Implementing OSPF