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The OSPFv3 protocol can be configured to participate in the checkpointing service, so that these protocols can execute a "graceful restart" when the management unit fails. In a graceful restart, the hardware to continues forwarding IPv6 packets using OSPFv3 routes while a backup switch takes over management unit responsibility Graceful restart uses the concept of "helpful neighbors". A fully adjacent router enters helper mode when it receives a link state announcement (LSA) from the restarting management unit indicating its intention of performing a graceful restart. In helper mode, a switch continues to advertise to the rest of the network that they have full adjacencies with the restarting router, thereby avoiding announcement of a topology change and and the potential for flooding of LSAs and shortest-path-first (SPF) runs (which determine OSPF routes). Helpful neighbors continue to forward packets through the restarting router. The restarting router relearns the network topology from its helpful neighbors.
Graceful restart can be enabled for either planned or unplanned restarts, or both. A planned restart is initiated by the operator through the management command initiate failover. The operator may initiate a failover in order to take the management unit out of service (for example, to address a partial hardware failure), to correct faulty system behavior which cannot be corrected through less severe management actions, or other reasons. An unplanned restart is an unexpected failover caused by a fatal hardware failure of the management unit or a software hang or crash on the management unit.
Use this command to enable the OSPF graceful restart functionality on an interface. To disable graceful restart, use the no form of the command.
Use this command to disable graceful restart for all restarts.
Use this command to configure the number of seconds that the restarting router asks its neighbors to wait before exiting helper mode. This is referred to as the grace period. The restarting router includes the grace period in its grace LSAs. For planned restarts (using the initiate failover command), the grace LSAs are sent prior to restarting the management unit, whereas for unplanned restarts, they are sent after reboot begins.
The grace period must be set long enough to allow the restarting router to reestablish all of its adjacencies and complete a full database exchange with each of those neighbors.
Use this command to revert the grace period to its default value.
Use this command to enable helpful neighbor functionality for the OSPF protocol. You can enable this functionality for planned or unplanned restarts, or both.
Use this command to disable helpful neighbor functionality for OSPF.
Use this command to disable helpful neighbor functionality for OSPF.
NOTE: The commands no nsf helper and nsf ietf helper disable are functionally equivalent. The command nsf ietf helper disable is supported solely for compatibility with other network software CLI.
The restarting router is unable to react to topology changes. In particular, the restarting router will not immediately update its forwarding table; therefore, a topology change may introduce forwarding loops or black holes that persist until the graceful restart completes. By exiting the graceful restart on a topology change, a router tries to eliminate the loops or black holes as quickly as possible by routing around the restarting router. A helpful neighbor considers a link down with the restarting router to be a topology change, regardless of the strict LSA checking configuration.
Use this command to require that an OSPF helpful neighbor exit helper mode whenever a topology change occurs.
Use this command to allow OSPF to continue as a helpful neighbor in spite of topology changes.