OSPFv3 Graceful Restart

When graceful restart is enabled, a routing device inform its neighbors when it is performing a restart.

Neighboring devices, known as graceful restart helpers, are informed by protocol extensions (grace-LSA) that the device is undergoing a restart and assist in the restart.

For the duration of the graceful restart, the restarting device and its neighbors continue forwarding packets, ensuring that there is no disruption to network performance or topology. Disruptions in forwarding are minimized and route flapping diminished.

When the restart is complete, the device can quickly resume full operation because of the assistance of the graceful restart helpers. The helper devices then return to normal operation.

There are two types of graceful restart:

Planned restart: The restarting routing device informs its neighbors before performing the restart. The graceful restart helpers act as if the routing device is still in the network topology, continuing to forward traffic to the restarting routing device. After a defined interval, a “grace period,” the neighbors consider the restart complete and the restarting routing device as part of the network topology again.

The graceful-restart command triggers a graceful restart. For more information, see Configure OSPFv3 Graceful Restart.
Unplanned restart: The routing device restarts without warning due to a software fault.

Note

Process restart takes precedence over graceful restart and non-stop routing (NSR).

Graceful restart helper

When OSPFv3 graceful restart helper is enabled on a device, the device enters helper mode when it receives a grace-LSA (link-state advertisement) from the OSPFv3 router for which graceful restart has been triggered. A grace-LSA informs helper devices that a router is about to gracefully restart its software.

A graceful restart helper device can come out of helper mode in the following scenarios:

The grace period has expired.
The helper mode has been disabled through the command line.
Topology changes occurred during the router restart.

For graceful restarts to succeed, OSPFv3 neighbors must have graceful restart helper enabled. Graceful restart helper is enabled by default when you start OSPFv3. For more information, see Configure OSPFv3 Graceful Restart Helper.