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Once upon a time, there was only one protocol for software-defined networking (SDN), and it was OpenFlow. Classic SDN depended on OpenFlow for communications between the SDN controller, the brains of the network, and the data plane devices that carried out its instructions.
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SDN has come to have a broader meaning, however, with increasing emphasis on centralized network virtualization and programmability, not just control/data plane separation. As this shift has occurred, other protocols have become important in the space. Cisco introduced an SDN protocol for automating propagation of policy through a network composed of smart devices rather than "blank slate" data plane devices. The rise of VMware NSX and other solutions has brought to prominence the VXLAN protocol for overlaying logical networks across existing networks. NVGRE is a similar virtualization protocol and is gaining prominence as Microsoft and others take advantage of it in their cloud environments. Geneve is an even newer virtualization protocol aimed at unifying VXLAN and NVGRE.
Given the growing number of choices, how can you decide which SDN protocol is best for you? For most, the answer is going to boil down to this: Identify which SDN solution can do what you want and need it to do in the next few years, and then use whichever protocol it supports. Certainly, OpenFlow has the broadest ecosystem of supporting vendors and technologies, but if you can't get what you need using a software-defined network based on OpenFlow, look elsewhere. In the longer run, expect a convergence of one or two protocols and migration paths to get from any deprecated protocols you might have implemented (perhaps NVGRE) to those replacing them (e.g., Geneve).
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