Ace the 2026 MEF SD-WAN Exam – Unlock the Future of Networking Mastery!

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What is the difference between underlay and overlay in MEF SD-WAN?

Underlay and overlay are the same concept; both refer to the software-defined network fabric.

Underlay is the physical/virtual transport network providing connectivity; Overlay is the SD-WAN fabric enabling application-aware routing, policy enforcement, encryption, and tunnel management across underlays.

In MEF SD-WAN, the underlay is the actual transport network that provides connectivity between sites—this covers the physical and virtual paths you use, such as MPLS, broadband Internet, or LTE links. The overlay is the SD-WAN fabric that sits on top of that transport, handling how traffic is routed, secured, and managed. It defines application-aware routing, policy enforcement, encryption, and tunnel management across the underlying underlays.

This is why the accurate choice describes the underlay as the connectivity fabric (physical/virtual transport) and the overlay as the SD-WAN layer that provides application-aware routing, policy enforcement, encryption, and tunnel management across underlays. The other statements misrepresent the roles or invert them.

Underlay is only security functions; Overlay is only monitoring.

Overlay provides physical transport; Underlay provides application routing.