SDN is an architecture, not a protocol: separate control plane from data plane, put the brain in a controller, program devices over APIs. Southbound: OpenFlow/NETCONF/RESTCONF; northbound: REST for applications. (“SDN = OpenFlow” is the classic false statement — OpenFlow is one implementation.)

• Devices become flow-table forwarders; the controller computes paths and installs entries.
• Design prerequisites: rock-solid routed underlay (OSPF/IS-IS/BGP), controller redundancy, API security, overlay segmentation (VRF/VXLAN).

Cisco’s lineup to know: DNA Center (campus automation), SD-Access (campus fabric), SD-WAN (WAN overlay), VXLAN (DC overlay), LISP (SD-Access control plane).

Exam mapping: campus fabric → SD-Access, WAN central policy → SD-WAN, controller pushing config → southbound API.