Cisco ASR 9000 Series Configuration Guide

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To ensure consistent databases after a restart, the OSPFv3 configuration must be identical to the
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.
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Although IPv6 FIB tables remain unchanged during a graceful restart, these tables eventually mark the
routes as stale through the use of a holddown timer. Enough time is allowed for the protocols to rebuild
state information and converge.
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The router on which OSPFv3 is restarting must send OSPFv3 hellos within the dead interval of the
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.
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Simultaneous graceful restart sessions on multiple routers are not supported on a single network segment.
If a router determines that multiple routers are in restart mode, it terminates any local graceful restart
operation.
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This feature utilizes the available support for changing the purge time of existing OSPFv3 routes in the
Routing Information Base (RIB). When graceful restart is enabled, the purge timer is set to 90 seconds
by default. If graceful restart is disabled, the purge timer setting is 0.
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This feature has an associated grace LSA. This link-scope LSA is type11.
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According to the RFC, the OSPFv3 process should flush all old, self-originated LSAs during a restart.
With the Graceful Restart feature, however, the router delays this flushing of unknown self-originated
LSAs during a graceful restart. OSPFv3 can learn new information and build new LSAs to replace the
old LSAs. When the delay is over, all old LSAs are flushed.
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If graceful restart is enabled, the adjacency creation time of all the neighbors is saved in the system
database (SysDB). The purpose for saving the creation time is so that OSPFv3 can use the original
adjacency creation time to display the uptime for that neighbor after the restart.
Warm Standby and Nonstop Routing for OSPF Version 2
OSPFv2 warm standby provides high availability across RP switchovers. With warm standby extensions,
each process running on the active RP has a corresponding standby process started on the standby RP. A
standby OSPF process can send and receive OSPF packets with no performance impact to the active OSPF
process.
Nonstop routing (NSR) allows an RP failover, process restart, or in-service upgrade to be invisible to peer
routers and ensures that there is minimal performance or processing impact. Routing protocol interactions
between routers are not impacted by NSR. NSR is built on the warm standby extensions. NSR alleviates the
requirement for Cisco NSF and IETF graceful restart protocol extensions.
It is recommended to set the hello timer interval to the default of 10 seconds. OSPF sessions may flap
during switchover if hello-interval timer configured is less then default value.
Note
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
352 OL-30423-03
Implementing OSPF
Warm Standby and Nonstop Routing for OSPF Version 2

Cisco ASR 9000 Series Specifications

General

Brand	Cisco
Model	ASR 9000 Series
Category	Network Router
Language	English