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Example: Configuring IS-IS for GRES with Graceful Restart

This example shows how to configure the Routing Engine’s graceful restart protocol extensions using the intermediate system to intermediate system (IS-IS) interior gateway protocol (IGP) to successfully enable graceful Routing Engine switchover (GRES) with graceful restart.


GRES prevents interruptions in network traffic if the primary Routing Engine fails when combined with either:

  • Graceful restart

  • Nonstop active routing (NSR)

Before you follow the directions here to configure graceful restart, be sure you have enabled GRES, which is disabled by default. See Configuring Graceful Routing Engine Switchover for more information.


If adjacencies between the Routing Engine and the neighboring peer 'helper' routers time out, graceful restart protocol extensions are unable to notify the peer 'helper' routers about the impending restart. Graceful restart can then stop and cause interruptions in traffic.

To ensure that these adjacencies are kept, change the hold-time for IS-IS protocols from the default of 27 seconds to a value higher than 40 seconds.

If your system uses the open shortest pathway first (OSPF) protocol instead of IS-IS, see Example: Configuring OSPF Timers for configuration information.


CLI Quick Configuration

To quickly configure the hold-time, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the different hierarchy levels shown.

Each interface must be set individually, with a value for each level that the routing device operates on. The minimum recommended value of 41 seconds is used in this example, your system may require a higher value based on size and traffic.

Level 1 and level 2 can be set to different values.

[edit protocols]

[edit logical-systems logical-system-name}

[edit logical-systems logical-system-name routing-instances routing-instance-name]

[edit routing-instances routing-instance-name]

Configuring the IS-IS Protocol Hold Time for Graceful Restart

Step-by-Step Procedure

To configure the IS-IS hold-time for graceful restart:

  1. Locate or set the interfaces.

  2. Set the network level and the hold-time in seconds for that level.

  3. If the routing device functions on more than one level, set the value for the other level.

  4. If you are done configuring the routing device, commit the configuration.


    Repeat the entire configuration on all routing devices in a shared network.



Verifying the IS-IS Protocol Hold Time for Graceful Restart


Verify that the IS-IS protocol hold-time is set to 41 seconds or greater to ensure that graceful restart is enabled.


Confirm your configuration by entering the show isis adjacency brief command from operational mode. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.


A high enough IS-IS protocol hold-time value allows your system configuration to restart and ensures that even if a Routing Engine fails, traffic continues.