Software Defined Networks do not always behave intuitively. In fact, depending on the configuration, they may seem to break the rules of traditional IP networks. Robust testing is the only reliable way to ensure they are working properly.
One significant change is the shift from hardware-only implementations to a combination of hardware and software. SDN and OpenFlow networks rely much more on software running on general-purpose servers. This means a certain amount of fundamental testing must be revisited.
APIs, protocols and vendor implementations must all be thoroughly tested to ensure compliance with standards, as well as, interoperability between various implementations. And new SDN applications change the behavior of the network. Validation testing should take place for each new application and each new application revision.