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

In the context of SD-WAN, the term "underlay" refers to the physical infrastructure that provides the foundation for the SD-WAN network. This infrastructure includes the physical connections such as routers, switches, and links (including MPLS, broadband, and LTE) that facilitate data transfer between different locations. The underlay is essential because it dictates the basic connectivity, performance, and reliability of the network.

The physical infrastructure is responsible for delivering data packets to their destination, regardless of how those packets are handled or secured by the overlay, which represents the virtualized layer that optimizes and manages the application traffic across the underlay. Understanding the role of the underlay is crucial for effective SD-WAN design, as performance relies heavily on the underlying physical transport mechanisms.