SDN architecture
Controller-based software-defined architecture: underlay, overlay and fabric, control/data-plane separation, and northbound/southbound APIs.
SDN separates the control plane (decisions: routing, policy) from the data plane (forwarding). In controller-based architecture the control plane centralizes in the controller while devices keep the data plane — the box still forwards packets, but its behavior is programmed from above.
Underlay, overlay, fabric
| Term | Definition |
|---|---|
| Underlay | The transport network delivering IP connectivity between fabric devices — often but not necessarily only physical infrastructure |
| Overlay | Virtual networks built on top (tunnels, e.g., VXLAN) — where tenants/services live |
| Fabric | Underlay + overlay + the control plane that operates them as one system |
Northbound versus southbound
| Interface | Direction | Examples/purpose |
|---|---|---|
| Northbound API (NBI) | Apps/tools → controller | REST APIs for automation, dashboards, integrations |
| Southbound API | Controller → devices | NETCONF/RESTCONF, OpenFlow, CLI/SNMP in practice — how intent is programmed |
Example: a network-as-a-service app calls the controller's northbound REST API to create a new segment. The controller decides which devices must change and programs them via southbound protocols. The app never touches a device CLI; the device never knows the app exists.
Related resources
Sources
- Cisco CCNA 200-301 v1.1 Exam Topics (official PDF) — reviewed 2026-11-02
Targets CCNA 200-301 v1.1 objectives. Supplemental blocks are labeled beyond exam scope.