If you already have IS-IS Authentication enabled and then downgrade to a previous release, the IS-IS adjacencies may not get established. This issue affects the 100 Gb 8606CQ links only, but it can result in traffic loss.
Note
This applies only when you downgrade the software from the current release to 6.1.x.
It does not apply when you downgrade the software to 6.2.x.
Use the following procedure as a workaround:
Disable IS-IS Authentication on 100 Gb ports on both peers.
Downgrade the software to the required release.
Re-enable IS-IS Authentication.