What Is SDN? Software-Defined Networking Explained
SDN (Software-Defined Networking) separates the network's control plane (the decisions) from the data plane (the forwarding), centralising the brains in a controller. Instead of configuring each device by hand, you program the whole network through the controller.
Control plane vs data plane
Traditionally every switch/router makes its own forwarding decisions and forwards traffic. SDN lifts the decision-making into a central controller with a full network view, leaving devices to simply forward as instructed. Result: consistent, centrally-managed, automatable networks.
Northbound and southbound APIs
The controller talks southbound (to devices, via NETCONF, OpenFlow) and northbound (to applications and automation, via REST APIs). This is the foundation of controller-based networking like Cisco DNA Center and SD-Access — and why network automation skills (Python, APIs) now sit alongside traditional CLI. A CCNA-introduced, CCNP-deepened topic.
Frequently asked questions
What is SDN?
Software-Defined Networking separates network control (decision-making) from forwarding, centralising the control plane in a programmable controller.
What is the difference between the control and data plane?
The control plane decides how traffic should flow; the data plane actually forwards packets. SDN centralises the control plane in a controller.
What are northbound and southbound APIs?
Southbound APIs connect the controller to network devices; northbound APIs connect it to applications and automation tools.
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