Operation mode differences in VDOMs
A VDOM, such as root, can have a maximum of 255 interfaces in Network Address Translation (NAT) mode or Transparent mode. This includes VLANs, other virtual interfaces, and physical interfaces. To have more than a total of 255 interfaces configured, you need multiple VDOMs with multiple interfaces on each.
In Transparent mode without VDOMs enabled, all interfaces on the FortiGate unit act as a bridge — all traffic coming in on one interface is sent back out on all the other interfaces. This effectively turns the FortiGate unit into a two interface unit no matter how many physical interfaces it has. When VDOMs are enabled, this allows you to determine how many interfaces to assign to a VDOM running in Transparent mode. If there are reasons for assigning more than two interfaces based on your network topology, you are able to. However, the benefit of VDOMs in this case is that you have the functionality of Transparent mode, but you can use interfaces for NAT/Route traffic as well.
You can add more VDOMs to separate groups of VLAN subinterfaces. When using a FortiGate unit to serve multiple organizations, this configuration simplifies administration because you see only the security policies and settings for the VDOM you are configuring.
One essential application of VDOMs is to prevent problems caused when a FortiGate unit is connected to a layer-2 switch that has a global MAC table. FortiGate units normally forward ARP requests to all interfaces, including VLAN subinterfaces. It is then possible for the switch to receive duplicate ARP packets on different VLANs. Some layer-2 switches reset when this happens. As ARP requests are only forwarded to interfaces in the same VDOM, you can solve this problem by creating a VDOM for each VLAN.
For more information about Transparent mode, see the Transparent Mode & Internal Segmentation Firewall (ISFW) handbook.\
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