A FortiGate is operating in NAT mode and configured with two virtual LAN (VLAN) sub interfaces added to the physical interface. Which statements about the VLAN sub interfaces can have the same VLAN ID, only if they have IP addresses in different subnets.

Within a single Virtual Domain (VDOM), each VLAN subinterface configured on the same physical parent interface must have a unique VLAN ID. The VLAN ID is the primary identifier that FortiGate uses to associate tagged traffic on a trunk link with the correct logical subinterface. Attempting to configure two subinterfaces on the same physical port with the same VLAN ID will result in a configuration error, as it would create ambiguity.

A. The uniqueness of the VLAN ID is a Layer 2 requirement, independent of the Layer 3 IP addressing scheme used on the subinterfaces.
C. This statement is incorrect because it is precisely the use of different VDOMs that allows the same VLAN ID to be used on the same physical interface.
D. The IP subnet configuration is irrelevant to the Layer 2 requirement that VLAN IDs must be unique on a per-physical-interface basis within a VDOM.

1. Fortinet Training Institute. (2021). NSE 4 Study Guide - FortiGate Infrastructure 7.0. "Layer 2 Switching and VLANs" section. "You can create multiple VLAN subinterfaces on a physical interface. Each subinterface must have a unique VLAN ID."

2. Fortinet. (2021). FortiOS Administration Guide 7.0.0. "Interfaces". p. 318. The configuration examples and principles demonstrate that each VLAN subinterface on a physical port is defined by its unique VLAN ID.