Chapter 19 Virtual Domains : Virtual Domains in Transparent mode : Transparent operation mode
  
Transparent operation mode
In transparent mode, the FortiGate unit becomes a layer-2 IP forwarding bridge. This means that Ethernet frames are forwarded based on destination MAC address, and no other routing is performed. All incoming traffic that is accepted by the firewall, is broadcast out on all interfaces.
In transparent mode the FortiGate unit is a forwarding bridge, not a switch. A switch can develop a port table and associated MAC addresses, so that it can bridge two ports to deliver the traffic instead of broadcasting to all ports. In transparent mode, the FortiGate unit does not following this switch behavior, but instead is the forwarding bridge that broadcasts all packets out over all interfaces, subject to security policies.
Features such as broadcast domains, forwarding domains, and STP apply to both FortiGate units and VDOMs in Transparent mode.