SD-WAN solutions provide a replacement for traditional WAN routers and are agnostic to WAN transport technologies. They provide dynamic, policy-based, application path selection across multiple WAN connections and supports service chaining for additional services such as WAN optimization and firewalls.
At their very core, SDN and SD-WAN solutions are based on the principle of separating the control plane from the data plane to make networking more intelligent. SD-WAN is also the basis of the new NFV (Network Function Virtualization) or Network Slicing feature of the 5G world.
In traditional WAN Optimization, traffic control is carried out by co-operating edge devices. However in a large deployment, manually configuring tens and hundreds of such equipment in diverse geo-locations is a challenge. In SDN / SD-WAN, the edge devices carry out these operations, but the decision on how this is to be done is taken in a central location.