OpenFlow is already the standard Southbound API in SDN field, but OpenFlow is the one of the many SouthBound approaches. In SDN solution, we don't necessarily need to use OpenFlow protocol to control data plane. RESTful API is another way to control or configure switches ( data plane ) if they supports. Arista Networks has provides Arista eAPI as RESTful control of switches. For more information in details, please refer to this article: http://blog.sflow.com/2013/08/restful-control-of-switches.html