Duplicate system IDs between adjacent IS-IS areas are not supported.
If a duplicate system ID exists between adjacent areas, the boundary node logs an event like the following:
1 2021-09-21T02:08:44.734-04:00 GB12 CP1 - 0x001dc705 - 00000000 GlobalRouter ISIS WARNING Duplicate System ID 0022.2222.2415 detected on Boundary Node between HostName:B15 Chassis MAC:b0:ad:aa:41:c0:00 (HOME area) and HostName:B14 Chassis MAC:b0:ad:aa:41:b8:00 (REMOTE area)
To recover from this event and ensure the information on the boundary node is correctly updated, you must perform the following tasks:
Change the system ID on both of the conflicting nodes.
Disable and re-enable IS-IS on both of the conflicting nodes.
Delete errors can display on the boundary node but they have no functional impact.