The control plane decides where traffic should go. In SDN, this logic moves from individual devices to a central controller.
Traditional: Runs on each device, executes routing protocols, builds local tables, limited visibility.
SDN: Runs on the controller, has complete topology visibility, computes optimal paths globally, pushes rules to all devices.
Benefits:
- Consistent policy enforcement
- Faster convergence after failures
- Simplified troubleshooting
- Easier traffic engineering
The controller becomes a single point of failure. You'll need redundancy.