In this illustration, we see three layers of SDN architecture, followed by a physical network. Starting at the top and working our way down, the first thing we see are the Network-control applications, which includes three routers representing "Routing," "Access Control," and "Load balancer." Below this is a dashed line, labeled "Northbound API" and separating the top layer with the middle layer.

The middle layer includes a router, which appears next to the label "SDN Controller (network operating system)." Routing, Access Control, and Load Balancer all connect to the SDN Controller, and the SDN Controller connects to switches, which appear in the lowest layer. Beneath the SDN Controller, however, is a dashed line labeled "Southbound API," which separates the middle layer from the lower layer.

The lower layer of the SDN architecture includes a system of five routers, labeled "SDN-Controlled Switches."

The top and middle layers of the architecture are designated as the "Control plane," while the lower layer is designated as the "Data plane."