Which underlay transport options are typical in SD-WAN, and how do they influence path selection?

• A. Underlay transport: Only MPLS is used; it has fixed characteristics and does not influence path selection.
• B. Underlay transport: Only satellite links are used and always preferred.
• C. MPLS, broadband Internet, and cellular (4G/5G); they influence path selection by offering different characteristics such as latency, bandwidth, reliability, and cost.
• D. Underlay transport has no effect on path selection.

Answer: (C)

Underlay transport options in SD-WAN are the networks that carry traffic behind the overlay. Typical ones include MPLS, broadband Internet, and cellular links. Each has different performance characteristics—latency, bandwidth, reliability, jitter, and cost—that the SD-WAN can monitor in real time. The path selection logic uses these metrics to decide which underlay to send a given application’s traffic on, often on a per-flow or per-application basis.

Real-time or latency-sensitive traffic (like voice or video) may be steered to the lower-latency path when available, while less sensitive or bulk traffic can use cheaper or higher-bandwidth paths. If a path degrades or fails, the SD-WAN can reroute traffic to another underlay to maintain performance, all while following configured policies and SLAs. This dynamic use of multiple underlays is why they influence path selection rather than being ignored or fixed.

Using a setup that relies on only one underlay and claims it won’t affect path choices isn’t consistent with how SD-WAN operates, and thinking that only one option (like satellite) is used or that underlay has no effect misses the core idea that different underlays drive the routing decisions.