Graceful Restart

IS-IS graceful restart, sometimes referred to as non-stop forwarding, provides for an IS-IS router to remain on the forwarding path during a restart of its IS-IS software. Graceful-restart has four elements to its configuration: enabling, helping a peer, database re-synchronization interval, and restart interval.

Enabling graceful restart instructs the firmware to perform a graceful restart, rather than a standard IS-IS restart. Restart is only initiated by a fail-over. Whether the failover is intentional or not, the failed router protocol is restarted on another module, and upon startup, IS-IS informs its neighbors using existing link aggregation groups. Use the graceful-restart enable command to enable the graceful restart ability on this router.

When the helper peer is informed that a graceful restart is taking place, it sends the restarting router its database and prevents the rest of the network from being informed there is an issue with the restarting router. The helper also monitors the network for topology changes. If no changes occur, the helper router continues to advertise as though no restart was occurring. If a topology change does occur, graceful restart is terminated on the restarting router and a standard restart occurs. Helper mode is enabled by default, but can be disabled on an IS-IS router using the no graceful-restart enable-help-peer command in IS-IS router command mode.

The length of time to allow database synchronization during a graceful restart can be configured using the graceful-restart restart-sync-interval command in IS-IS router configuration mode.

A restart interval provides for a maximum time in seconds after which the graceful restart will terminate should it not complete or terminate for other reasons within the interval. Use the graceful-restart restart-interval command to change the restart interval setting.

View the router OSPF section of the show running-config display to verify any non-default graceful restart settings.