Configuring Graceful Restart

To enable IS-IS graceful restart (also known as nonstop forwarding, or NSF) on the router, you must first issue the nsf ietf command (in Router Configuration mode). You can then configure one or more optional timing parameters for graceful restart on the router.

To enable IS-IS graceful restart and configure optional graceful restart parameters:

1. Specify a previously configured IS-IS routing process to access Router Configuration mode. (For information about enabling IS-IS on the router, see Enabling IS-IS for IP Routing.)

   host1(config)#router isis engineering

   host1(config-router)#

2. Enable the IS-IS graceful restart mechanism for the router.

   host1(config-router)#nsf ietf

3. (Optional) Configure one or more of the following timing parameters for the restarting router:
   • Set the maximum time in seconds that the router waits before completing the restart process.

     host1(config-router)#nsf interface wait 30

   • Set the time interval in seconds between restart requests sent by the router.

     host1(config-router)#nsf t1 interval 60

   • Set the number of times that the router resends unacknowledged restart requests.

     host1(config-router)#nsf t1 retry-times 3

   • Set the maximum time in seconds that the router waits for the LSP database to synchronize. You must configure this parameter separately for each IS-IS level at which the router operates.

     host1(config-router)#nsf t2 level-1 70

     host1(config-router)#nsf t2 level-2 50

   • Set the maximum time in seconds that the restarting router waits before setting the overload bit to indicate that the graceful restart operation has failed. You can use either of the following methods:
     • Set the wait time manually to the specified number of seconds.

       host1(config-router)#nsf t3 manual 80
       

     • Specify that router obtain the wait time from neighboring IS-IS routers to which it has active adjacencies.

       host1(config-router)#nsf t3 adjacency

4. (Optional) Issue the show isis nsf command from Privileged Exec mode to verify the graceful restart configuration.

   host1(config-router)#exit

   host1(config)#exit

   host1#show isis nsf

   For more information about monitoring graceful restart, see the show isis nsf command description in Monitoring IS-IS Parameters and the show clns neighbors detail command description in Displaying CLNS .

nsf ietf

• Use to enable the IS-IS graceful restart mechanism on the router.
• Graceful restart, which is also known as nonstop forwarding (NSF), allows an IS-IS router to restart with minimal routing disruption to the network.
• Example

  host1(config-router)#nsf ietf

• Use the no version to restore the default state for IS-IS graceful restart on the router, disabled.
• See nsf ietf

nsf interface wait

• Use to specify the maximum amount of time, in seconds, that an IS-IS process on a restarting router waits for all interfaces with IS-IS adjacencies to come up before completing the restart process.
• You can specify a value in the range 5–120 seconds.
• Example

  host1(config-router)#nsf interface wait 45

• Use the no version to restore the default maximum wait time, 10 seconds.
• See nsf interface wait

nsf t1

• Use to specify either the interval between IS-IS restart requests sent by the router or the number of times that the router resends unacknowledged restart requests.
• Use the interval keyword to specify the number of seconds, in the range 5–120, between restart requests sent by the router on a particular IS-IS interface to neighboring IS-IS routers in the network.
• Use the retry-times keyword to specify the number of times, in the range 1–3, that the router tries to resend unacknowledged restart requests.
• The restarting router stops sending restart requests after it receives an acknowledgment.
• Example 1

  host1(config-router)#nsf t1 interval 90

• Example 2

  host1(config-router)#nsf t1 retry-times 2

• Use the no version to restore the default time interval, 5 seconds, or the default number of retry attempts, 1.
• See nsf t1

nsf t2

• Use to specify the maximum amount of time, in seconds, that a restarting router waits for the LSP database to synchronize.
• You must configure independent instances of the T2 timer for each IS-IS level at which the router operates. This requirement means that for a level 1-2 router, you must issue this command twice: first to configure the timer for level 1, and a second time to configure it for level 2.
• Use either the level-1 keyword to set the T2 wait time for level 1 routing, or the level-2 keyword to set the wait time for level 2 routing.
• You can specify a value in the range 5–120 seconds for each level.
• Example—Configures the T2 wait time for a level 1-2 IS-IS router

  host1(config-router)#nsf t2 level-1 70

  host1(config-router)#nsf t2 level-2 50

• Use the no version to restore the default T2 wait time, 30 seconds.
• See nsf t2

nsf t3

• Use to specify the maximum amount of time, in seconds, that the restarting router waits before setting the overload bit.
• The restarting router sets the overload bit to indicate that the LSP database has not been synchronized and the IS-IS graceful restart operation has failed.
• You must use one of the following methods to set the T3 wait time:
  • Use the manual keyword and a value in the range 5–120 seconds to set the T3 wait time manually.
  • Use the adjacency keyword to specify that the restarting router should obtain its T3 wait time from neighboring IS-IS routers that have active adjacencies to this router. This option sets the wait time to the minimum of the remaining times specified in the restart TLVs contained in the hello packets that the router receives from its neighbors.
• Example1

  host1(config-router)#nsf t3 manual 120

• Example 2

  host1(config-router)#nsf t3 adjacency

• Use the no version to restore the default T3 wait time, 30 seconds.
• See nsf t3

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