Fortinet Fortigate-5000 series - Rules for VLAN Ids; Transparent Mode Virtual Domains and Vlans

System Network VLANs in Transparent mode
FortiGate Version 3.0 MR4 Administration Guide
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Figure 50: FortiGate unit in Transparent mode
Rules for VLAN IDs
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 the
same VLAN ID. Their relationship is the same as the relationship between any
two FortiGate network interfaces.
Transparent mode virtual domains and VLANs
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 “Using virtual domains” on page 61
Internet
Untagged packets
Router
VLAN 1
VLAN 2
VLAN 3
VLAN 1 Network
VLAN 1
VLAN Switch
VLAN Trunk
FortiGate unit
in Transparent mode
VLAN Switch
VLAN 1
VLAN 2
VLAN 3
VLAN Trunk
VLAN 3 Network
VLAN 3
VLAN 2 Network
VLAN 2
Note: There is a maximum of 255 VLANs allowed per interface in Transparent mode.