Fortinet Secure SD-WAN uses underlay and overlay networks to separate transport connectivity from logical routing and secure communication between enterprise locations.
The underlay network provides the underlying IP transport, including broadband internet, MPLS, leased lines, and wireless connectivity such as 4G/5G LTE. The overlay network provides logical connectivity through tunnels established over those transport networks.
Option A is correct because wireless connections can serve as the underlying transport for overlay tunnels. For example, a FortiGate with FortiExtender can establish an IPsec VPN overlay over a 5G cellular WAN connection.
Option E is correct because overlay networks provide routing flexibility independently of the physical transport infrastructure. IPsec VPN and ADVPN overlays enable dynamic route exchange, secure branch connectivity, path selection, and shortcut establishment.
Option B is incorrect because underlay connectivity is not restricted to wired interfaces. Cellular WAN connectivity is fully supported.
Option C is incorrect in the Fortinet SD-WAN terminology used here. A VLAN provides Layer 2 logical segmentation and is not inherently an SD-WAN overlay tunnel.
Option D is incorrect because FortiLink primarily provides management and connectivity between FortiGate and FortiSwitch devices. It is not classified as an SD-WAN transport underlay solely because it is a network interface.
[References: FortiOS 7.6 Administration Guide — SD-WAN Components and Design Principles; Fortinet SD-WAN Architecture for Enterprise — Underlay, Overlay, and FortiExtender., , , ]
