In MEF SD-WAN, what is the relationship between the underlay and the overlay?

• A. The underlay provides the physical or transport connectivity; the overlay uses tunnels and policies to route traffic over one or more underlays.
• B. The overlay is independent of the underlay and does not use tunnels.
• C. The underlay handles application QoS independently of the overlay.
• D. The underlay is only used for management traffic.

Answer: (A)

In MEF SD-WAN, the network is divided into two layers: the underlay and the overlay. The underlay provides the actual physical or transport connectivity—the real paths that connect devices across networks. The overlay sits on top of that transport and uses tunnels to create virtual networks over one or more underlays, applying policies to route traffic, enforce QoS, and support features like encryption and segmentation.

This separation lets you mix different transport technologies (MPLS, Internet links, wireless, etc.) and still have a single, policy-driven WAN. The overlay encapsulates traffic, builds the logical tunnels, and applies application-aware decisions, while the underlay simply carries that traffic between endpoints. It’s not correct to think the overlay operates without tunnels or that the underlay is only for management traffic—the underlay carries user data, and the overlay relies on those tunnels and policies to function across the underlying transport.