Solution | Root cause: The VLAN ID in question is being reused across two different FortiLink interfaces. The switch template involved is referencing a VLAN object that is actually associated with a different FortiLink interface than the one connected to the switch. Because a VLAN object can only be tied to one FortiLink interface at a time, FortiManager rejects the install with a 'VLAN vlanname exists on device and is in use under a different interface' error.
Identify the conflicting VLAN from the install log.
The error message includes an object reference ID (e.g. id=16847), not the VLAN number itself. On FortiManager, go to the object it references: Policy & Objects -> Object Configurations (or FortiLink -> Interface). Use this ID to look up which VLAN name/ID is in conflict, and which FortiLink interface it is currently tied to.
Identify the FortiLink interface connected to the switch.
In FortiManager, go to Device Manager, select the respective FortiGate on which the install fails, select the Network tab, and identify the FortiLink interface name. (Optional but recommended) Verify current VLAN-to-FortiLink mapping on the FortiGate.
In the GUI: Log in to the FortiGate directly (or use FortiManager's Device Manager -> CLI Console for that device) and go to WiFi & Switch Controller -> FortiSwitch VLANs to see which FortiLink interface each VLAN is currently assigned to. Edit the switch template.
Go to FortiSwitch -> Switch Template, open the template used by the affected switch, and under Port Settings, set the VLAN to the VLAN created for the FortiLink interface identified in Step 2 (not the one it currently points to).
Apply and reinstall.
Apply the changes to the switch template, then reinstall the configuration from FortiManager to FortiGate (Device Manager -> Install Wizard).
If the issue persists:
Re-check the install log for the same or a different conflicting VLAN reference, and confirm there is no second VLAN with the same conflict on another FortiLink interface. Repeat Steps 1 - 6 for each conflict found.
Reference - Example install log:
Also, details of the error can be seen on the below debug outputs.
diagnose debug enable
diagnose debug application securityconsole 255
"fsp managed-switch", "", id=16847, ERROR - VLAN vlanname exists on device and is in use under a different interface "fsp vlan", "", id=19762, ERROR - VLAN vlanname exists on device and is in use under a different interface
"switch-controller managed-switch ports", "port2", id=19769, ERROR - VLAN vlanname exists on device and is in use under a different interface
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