Fortinet fortigate-200a Betriebsanweisung

In Transparent mode two VLAN subinterfaces added to the same physical interface 
cannot have the same VLAN ID. However, you can add two or more VLAN 
subinterfaces with the same VLAN IDs to different physical interfaces. There is no 
internal connection or link between two VLAN subinterfaces with same VLAN ID. Their 
relationship is the same as the relationship between any two FortiGate network 
interfaces.

VLAN subinterfaces are added to and associated with virtual domains. By default the 
FortiGate configuration includes one virtual domain, named root, and you can add as 
many VLAN subinterfaces as you require to this virtual domain.

You can add more virtual domains if you want to separate groups of VLAN 
subinterfaces into virtual domains. For information on adding and configuring virtual 
domains, see