According to theFortiOS 7.6 Administration Guideand theFortiSwitch 7.6 FortiLink Guide, deploying managed switches over a Layer 3 underlay—such as the public internet—requires a specific tunneling mechanism to bridge Layer 2 broadcast domains. Traditional FortiLink relies on a direct Layer 2 connection; however, for remote sites,FortiLink over VXLANis the standard solution.

FortiLink over VXLAN (Option A):Virtual Extensible LAN (VXLAN) is used to encapsulate Layer 2 Ethernet frames into Layer 3 UDP packets, allowing VLAN-tagged traffic to traverse an ISP ' s routable network. This enables the FortiGate to manage remote FortiSwitch " islands " as if they were locally connected, maintaining full VLAN segmentation across the WAN.

Layer 3 Termination (Option E):The FortiGate acts as theVirtual Tunnel Endpoint (VTEP). It must have a reachable Layer 3 interface (such as a WAN port with a public IP or an IPsec tunnel interface) to terminate the VXLAN overlay. Once the VXLAN tunnel is terminated at the FortiGate, the encapsulated VLAN traffic is extracted, and the FortiGate can perform inter-VLAN routing and security inspection.

Regarding the incorrect options:Option Bis incorrect because the FortiGate at the central site handles the routing, eliminating the need for a local L3 device.Option Cis a performance consideration but not a functional requirement for basic connectivity.Option Dis often used for security to encrypt the underlay, but IPsec alone does not provide the Layer 2 extension capabilities required for VLAN segmentation; VXLAN is the specific component that handles the MAC-in-UDP encapsulation.