What is SDN (Software-Defined Networking)?
Why Interviewers Ask This
This is a differentiating question used for senior and lead roles. Interviewers want to see if you can explain not just what happens, but why — and what the trade-offs are in different approaches.
Answer
SDN (Software-Defined Networking) decouples the network's control plane (brain — deciding where traffic goes) from the data plane (muscle — forwarding packets), centralizing control in software. Traditional networks are distributed — each router/switch makes its own decisions using distributed protocols (OSPF, STP). In SDN, a centralized controller has a global view of the network and programs forwarding rules into network devices via southbound APIs (OpenFlow being the standard protocol). Applications interact with the controller via northbound APIs. Benefits: programmability (automate network changes via code), centralized visibility (single view of the entire network), agility (rapid policy changes without touching individual devices), vendor neutrality. SDN is the foundation for: network virtualization (NSX, ACI), cloud networking (AWS VPC, Azure VNet), and network automation. OpenDaylight, ONOS, and Cisco ACI are SDN controller implementations.
Pro Tip
Back up your answer with a specific project or situation. Saying 'In my last Networking project, I used this when...' immediately makes your answer more credible and memorable.