Chapter 19 Virtual Domains : Inter-VDOM routing : Benefits of inter-VDOM routing : Configuration flexibility
  
Configuration flexibility
A typical VDOM uses at least two interfaces, typically physical interfaces, one for internal and one for external traffic. Depending on the configuration, more interfaces may be required. This means that the maximum number of VDOMs configurable on a FortiGate unit using physical interfaces is the number of interfaces available divided by two. VLANs can increase the number by providing multiple virtual interfaces over a single physical interface, but VLANs have some limitations. Using physical interfaces for inter-VDOM communication therefore limits the number of possible configurations on your FortiGate unit.
To overcome this limitation, inter-VDOM links can be created within the FortiGate unit. Using virtual interfaces, inter-VDOM links free up the physical interfaces for external traffic. Using VDOM links on a FortiGate unit with 8 physical interfaces, you can have 4 VDOMs communicating with each other (meshed configuration) and continue to have 2 physical interfaces each for internal and external connections. This configuration would have required 20 physical interfaces without inter-VDOM routing. With inter-VDOM routing it only requires 8 physical interfaces, with the other 12 interfaces being internal VDOM links.
Inter-VDOM routing allows you to make use of Standalone VDOM configuration, Management VDOM configuration and Meshed VDOM configuration without being limited by the number of physical interfaces on your FortiGate unit.