Software-defined networking uses an operation mode that is sometimes called adaptive or dynamic, in which a switch issues a route request to a controller for a packet that does not have a specific route.
This process is separate from adaptive routing, which issues route requests through routers and algorithms based on the network topology, not through a controller.
With SDN, the administrator can change any network switch’s rules when necessary — prioritizing, de-prioritizing or even blocking specific types of packets with a very granular level of control. This is especially helpful in a cloud computing multi-tenant architecture, because it allows the administrator to manage traffic loads in a flexible and more efficient manner.
Essentially, this allows the administrator to use less expensive commodity switches and have more control over network traffic flow than ever before.
Where a traditional network would use a specialized appliance such as a firewall or link-load balancer, an SDN deploys an application that uses the controller to manage data plane behavior. Applications talk to the controller though its north-bound interface.