Which statement correctly describes the underlay in MEF SD-WAN?

• A. The underlay refers to the overlay fabric's configuration.
• B. The underlay is the policy management layer.
• C. The underlay is the physical/virtual transport network providing connectivity, such as MPLS, Internet, and LTE/5G.
• D. The underlay is the application layer on top of the network.

Answer: (C)

Underlay refers to the physical and virtual transport network that provides the actual connectivity between sites. In MEF SD-WAN, this is the substrate that carries traffic and can include multiple paths such as MPLS, the Internet, and wireless links like LTE or 5G. The overlay sits on top of this underlay, building the logical SD-WAN fabric with tunnels, routing decisions, and policies that steer traffic across the diverse transport paths. The other layers—policy management or app-layer functionality—are separate from the transport substrate, making the underlay specifically about the connectivity backbone rather than the overlay fabric or higher-level management and applications.