Tying up physical interfaces on the FortiGate unit presents a problem. With a limited number of interfaces available, configuration options for the old style of communication between VDOMs are very limited. VLANs can be an answer to this, but they have some limitations.

For example, the FortiGate-800 has 8 physical ethernet ports. If they are assigned 2 per VDOM (one each for external and internal traffic) there can only be 4 VDOMs at most configured, not the 10 VDOMs the license will allow. Adding even one additional interface per VDOM to be used to communicate between VDOMs leaves only 2 VDOMs for that configuration, since it would required 9 interfaces for 3 VDOMs. Even using one physical interface for both external traffic and inter-VDOM communication would severely lower the available bandwidth for external traffic on that interface.

With the introduction of inter-VDOM routing, traffic can travel between VDOMs internally, freeing up physical interfaces for external traffic. Using the above example we can use the 4 VDOM configuration and all the interfaces will have their full bandwidth.