This article offers solutions and guidance on resolving ISIS adjacency not forming issues after upgrading to Junos OS 19.2R1-S2

ISIS adjacencies fail to establish following an upgrade to Junos OS 19.2R1-S2 or newer releases

The ISIS adjacency issue that occurs after upgrading to Junos OS 19.2R1-S2 or newer releases is attributed to a change in the ISIS PDU exchange behavior. In earlier Junos versions, ISIS PDU exchange functioned without the need for explicit configuration of the packet mode for ISIS. However, in newer Junos releases, this setting is essential for proper ISIS PDU exchange. To resolve the adjacency problem, ensure that you configure the packet mode for ISIS when upgrading to Junos OS 19.2R1-S2 or newer releases. Failure to do so will result in ISIS adjacencies not forming as expected.

To enable the packet mode for ISIS, please refer to the following command:

set security forwarding-options family iso mode packet-based

Note: The above configuration will only change the ISO mode to packet-based, and the rest of the firewall functionality will continue to work in flow-mode.