Graceful Shutdown For Ospfv3; Modes Of Graceful Restart Operation; Restart Mode - Cisco ASR 9000 Series Configuration Manual

Implementing OSPF
cards to remain up through a failover and to be kept current with the Forwarding Information Base (FIB) on
the active RP is key to Cisco IOS XR Software NSF operation.
Routing protocols, such as OSPF, run only on the active RP or DRP and receive routing updates from their
neighbor routers. When an OSPF NSF-capable router performs an RP failover, it must perform two tasks to
resynchronize its link-state database with its OSPF neighbors. First, it must relearn the available OSPF
neighbors on the network without causing a reset of the neighbor relationship. Second, it must reacquire the
contents of the link-state database for the network.
As quickly as possible after an RP failover, the NSF-capable router sends an OSPF NSF signal to neighboring
NSF-aware devices. This signal is in the form of a link-local LSA generated by the failed-over router. Neighbor
networking devices recognize this signal as a cue that the neighbor relationship with this router should not be
reset. As the NSF-capable router receives signals from other routers on the network, it can begin to rebuild
its neighbor list.
After neighbor relationships are reestablished, the NSF-capable router begins to resynchronize its database
with all of its NSF-aware neighbors. At this point, the routing information is exchanged between the OSPF
neighbors. After this exchange is completed, the NSF-capable device uses the routing information to remove
stale routes, update the RIB, and update the FIB with the new forwarding information. OSPF on the router
and the OSPF neighbors are now fully converged.

Graceful Shutdown for OSPFv3

The OSPFv3 Graceful Shutdown feature preserves the data plane capability in these circumstances:
• RP failure resulting in a switch-over to the backup processor
• Planned OSPFv3 process restart, such as a restart resulting from a software upgrade or downgrade
• Unplanned OSPFv3 process restart, such as a restart resulting from a process crash
In addition, OSPFv3 will unilaterally shutdown and enter the exited state when a critical memory event,
indicating the processor is critically low on available memory, is received from the sysmon watch dog process.
This feature supports nonstop data forwarding on established routes while the OSPFv3 routing protocol restarts.
Therefore, this feature enhances high availability of IPv6 forwarding.

Modes of Graceful Restart Operation

The operational modes that a router can be in for this feature are restart mode, helper mode, and protocol
shutdown mode.

Restart Mode

When the OSPFv3 process starts up, it determines whether it must attempt a graceful restart. The determination
is based on whether graceful restart was previously enabled. (OSPFv3 does not attempt a graceful restart upon
the first-time startup of the router.) When OSPFv3 graceful restart is enabled, it changes the purge timer in
the RIB to a nonzero value. See
to enable and configure graceful restart.
During a graceful restart, the router does not populate OSPFv3 routes in the RIB. It tries to bring up full
adjacencies with the fully adjacent neighbors that OSPFv3 had before the restart. Eventually, the OSPFv3
process indicates to the RIB that it has converged, either for the purpose of terminating the graceful restart
(for any reason) or because it has completed the graceful restart.
OL-30423-03
Configuring OSPFv3 Graceful Restart, on page
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
Graceful Shutdown for OSPFv3
387,for descriptions of how
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