What is required for two VLAN sub-interfaces on the same physical interface in NAT/Route mode?

In NAT/Route mode, each VLAN sub-interface needs a unique VLAN ID to ensure proper packet routing and to avoid conflicts on the physical interface. When multiple VLANs are configured on a single physical interface, they must be distinctly identified with different VLAN IDs. This distinction allows the Fortinet device to differentiate between the VLAN traffic at Layer 2, facilitating the correct handling and routing of packets associated with each VLAN.

Utilizing the same VLAN ID for multiple sub-interfaces can create confusion and routing issues, as the device would not be able to properly distinguish the traffic originating from different VLANs. This could lead to misdelivered packets, security vulnerabilities, and, ultimately, network dysfunction.

Other options suggest conditions under which VLAN IDs could be the same, such as being in different VDOMs or connected to different switches. However, these scenarios do not apply, as VLAN segmentation relies on unique identifiers to segregate traffic successfully. Thus, the requirement for unique VLAN IDs for each sub-interface remains upheld to ensure proper network functionality.