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Configuration Guide - IP Unicast Routing

S1720, S2700, S5700, and S6720 V200R011C10

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Enabling IS-IS GR

Enabling IS-IS GR


The restart of an IS-IS switch causes the temporary interruption of the network, because the adjacency relationship between the switch and its neighbor is torn down. The LSPs of the switch are deleted, which makes route calculation inaccurate. Packets are thus lost.

You can configure IS-IS GR to solve this problem. After IS-IS GR is enabled, the switch notifies the neighbor of the restart status, and reestablishes the adjacency relationship with its neighbor without interrupting the forwarding.

The GR interval is set as the Holdtime in an IS-IS Hello PDU. Therefore, the adjacency relationship is not torn down when the switch restarts.


  1. Run system-view

    The system view is displayed.

  2. Run isis [ process-id ]

    The IS-IS view is displayed.

  3. Run graceful-restart

    IS-IS GR is enabled.

    By default, IS-IS GR is disabled.

  4. Run graceful-restart no-impact-holdtime

    The holdtime of an IS-IS neighbor is configured to remain unchanged in IS-IS GR mode.

  5. (Optional) Run graceful-restart interval interval-value

    A value is configured for the T3 timer during the IS-IS GR.

    The value of the T3 timer indicates the longest time that a GR lasts. A router disables the T3 timer after the LSDB synchronization ends in all areas. If LSDBs are not synchronized yet when the T3 timer expires, the GR fails.

    By default, the value of the T3 timer is 300 seconds. Keeping the default value is recommended.

    During a GR, an IS-IS neighbor of the restarter sets the value of the T3 timer to the holdtime of the neighbor relationship between them, which prevents routes from being recalculated on the whole network due to a neighbor disconnection during the GR.

  6. (Optional) Run graceful-restart suppress-sa

    The GR restarter is configured to suppress the Suppress-Advertisement (SA) bit of the restart TLV.

    By default, the SA bit is not suppressed.

Verifying the Configuration

Run display isis graceful-restart status [ level-1 | level-2 ] [ process-id | vpn-instance vpn-instance-name ] command to check the status of IS-IS GR.

Updated: 2019-10-21

Document ID: EDOC1000178171

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