Define the SD-WAN overlay and explain how it differs from the underlay.

• A. Overlay is the physical transport; Underlay is the virtual fabric
• B. Overlay is the virtual fabric carrying traffic via tunnels with policy-driven routing; Underlay is the physical transport (MPLS, Internet, LTE) and its routing
• C. Overlay is used only for management traffic
• D. Overlay and underlay are the same

Answer: (B)

In SD-WAN, the overlay is the virtual network that carries traffic across the WAN through tunnels, using centralized, policy-driven routing to decide how to forward applications between sites. The underlay is the actual physical transport—MPLS, Internet, and LTE—and the basic routing that moves packets between devices on the physical network. The overlay decouples traffic decisions from the physical links, so you can steer traffic based on application needs and policies regardless of whether a path uses MPLS, broadband Internet, or cellular.

That’s why the best description is that the overlay is the virtual fabric carrying traffic via tunnels with policy-driven routing, while the underlay is the physical transport and its routing. The other options mischaracterize the roles (overlay as physical, or as only management traffic, or as the same as the underlay).