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How Graceful Restart Works

Graceful restart is disabled on the router by default. When you enable graceful restart by issuing the nsf ietf command, the router sends restart requests to neighboring routers to notify them that it is restarting. The restarting router includes the restart TLV (type 211) in its hello PDUs to signal the other routers that it supports graceful restart and to request help resynchronizing its LSP database. Including the restart TLV in hello packets also ensures that neighboring routers will maintain their active adjacencies to the restarting router and keep the restarting router in the network topology.

Graceful restart uses a set of configurable timers to support the restart mechanism. Table 15 briefly describes these timers and lists the associated commands that you can use to configure the timer values on the router.

Table 15: IS-IS Graceful Restart Timers

Timer	Description	Associated Command
Interface wait	Sets the maximum time (in seconds) that the router waits for all interfaces with IS-IS adjacencies to come up before completing the restart process	nsf interface wait
T1	Sets the time interval (in seconds) between restart requests sent by the router, and the number of times that the router resends unacknowledged restart requests	nsf t1
T2	Sets the maximum time (in seconds) that the router waits for the LSP database to synchronize	nsf t2
T3	Sets the maximum time (in seconds) that the restarting router waits before setting the overload bit to indicate that graceful restart has failed	nsf t3

For details about configuring graceful restart, see Configuring Graceful Restart .

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