Physical switch selection is always an interesting question to ask when building your network. The term 'switch' in this context should be broadly interpreted as both L2 and L3 devices. In the SDN world, where we separate the control and data plane, any network function, such as routing, switching, firewall, monitoring, or firewall function, are software function while we reduce the physical packet transport into flows. The line between software-based versus hardware switch is almost non-existent when it comes to management and control plane constructs. However, we still need to be concerned with hardware capacity when it comes to building our network infrastructure.

If you draw a comparison between network engineering and systems engineering when purchasing servers, a system engineer would consider hardware specifications, such as CPU, memory, and storage, while separating the software aspects of operating system and software packages. The two sides of hardware and software still need to work together, though the industry has matured enough that for the most part the interoperability is not a big issue.

Network engineering with SDN and OpenFlow is moving toward that direction, but is not as matured. When choosing a physical switch, besides considering the size, port count, and number of programmable flows, it is always a good idea to choose switches that have gone through some testing with the SDN technology in mind. In our use case, we shoud take a closer look at switches with stated OpenFlow specification support, and perhaps consulting vendor-neutral lab testing results, such as InCNTRE from Indiana University (http://incntre.iu.edu/SDNlab). More specifically, controller projects such as Ryu and commercial controllers such as BigSwitch and Cisco ONE, all provide their own testing results.
In the years since the first introduction of OpenFlow, the market for physical switch selection has grown tremendously. I have picked three categories and representative devices to help you in your own evaluation.