Graceful Restart Requirements And Restrictions - Cisco NCS 6000 Series Configuration Manual

Protocol Restoration
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
• 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.
• To ensure consistent databases after a restart, the OSPFv3 configuration must be identical to the
• Although IPv6 FIB tables remain unchanged during a graceful restart, these tables eventually mark the
• The router on which OSPFv3 is restarting must send OSPFv3 hellos within the dead interval of the
• Simultaneous graceful restart sessions on multiple routers are not supported on a single network segment.
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
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is restarting, each router is called a helper.
configuration before the restart. (This requirement applies to self-originated information in the local
database.) A graceful restart can fail if configurations change during the operation. In this case, data
forwarding would be affected. OSPFv3 resumes operation by regenerating all its LSAs and resynchronizing
its database with all its neighbors.
routes as stale through the use of a holddown timer. Enough time is allowed for the protocols to rebuild
state information and converge.
process restart. Protocols must be able to retain adjacencies with neighbors before the adjacency dead
timer expires. The default for the dead timer is 40 seconds. If hellos do not arrive on the adjacency before
the dead timer expires, the router takes down the adjacency. The OSPFv3 Graceful Restart feature does
not function properly if the dead timer is configured to be less than the time required to send hellos after
the OSPFv3 process restarts.
If a router determines that multiple routers are in restart mode, it terminates any local graceful restart
operation.
Implementing OSPF