In SD-WAN terminology, what does the underlay refer to?

• A. The physical or logical transport network over which overlay tunnels travel.
• B. The application layer protocol.
• C. The virtual machine hosting the edge.
• D. The security policy.

Answer: (A)

Underlay refers to the actual transport network that carries the SD-WAN overlays. It includes the physical or logical links—such as broadband Internet, MPLS, fiber, or other networks—and the basic connectivity and performance characteristics (bandwidth, latency, jitter, loss, QoS) that enable the overlay tunnels to function. The SD-WAN overlay sits on top of this underlay, building tunnels (like IPsec or other encapsulations) to move application traffic between edge devices, while the overlay handles path selection, policy, and application-aware routing. So, the underlay is the network infrastructure that provides the raw transport, on which the overlay tunnels travel. The other options describe layers or elements not representing the transport network itself: the application layer protocol, the virtual machine hosting the edge, and a security policy.