Configuring IS-IS GR

By configuring IS-IS GR, you can enable Router to restart gracefully and avoid temporary black holes.

Context

The restart of an IS-IS router causes the temporary interruption of the network, because the adjacency relationship between the router and its neighbor is torn down. The LSPs packets of the router 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 router notifies the neighbor of the restart status, and reestablishes the adjacency relationship with its neighbor without interrupting the forwarding.

The advantages of IS-IS GR are as follows:

When IS-IS restarts, the router can resend connection requests to its neighbor. The adjacency relationship is not torn down.
Before LSPs packets are generated, GR minimizes the interference caused by waiting for the database synchronization.
If the router starts for the first time, the router sets the overload bit in LSPs until the LSDB synchronization is complete. This avoids route black holes.

Procedure

Access the system view.
system-view
Access the IS-IS process view.
isis [ process-id ]
Enable IS-IS GR.
graceful-restart

By default, IS-IS GR is disabled.
Set the holdtime of an IS-IS neighbor to remain unchanged in IS-IS GR mode.
graceful-restart no-impact-holdtime
(Optional) Set the T2 timer during the IS-IS GR.
graceful-restart t2-interval interval-value

The value of the T2 timer indicates the longest time during which the system waits for the LSDB synchronization. Each Level-1 or Level-2 router maintains a T2 timer and disables it after the LSDB synchronization among Level-1 or Level-2 routers ends. If LSDBs are not synchronized yet when the T2 timer expires, the GR fails.

By default, the value of the T2 timer is 60 seconds. Keeping the default value is recommended.
(Optional) Set the T3 timer during the IS-IS GR.
graceful-restart interval interval-value

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.
(Optional) Configure the GR restarter to suppress the Suppress-Advertisement (SA) bit of the restart TLV.
graceful-restart suppress-sa

The SA bit determines whether a neighbor (GR helper) advertises the neighbor relationship with the restarter. The helper suppresses the advertisement of the neighbor relationship with the restarter until the helper receives a packet in which SA is set to 0. By default, the SA bit is not suppressed.