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How many software-defined networking controllers do you need? Good question, and there are a few things we need to keep in mind. The software-defined networking controller's purpose is to provide ease of management for flow control, meaning it can be used to push policy for automated policy control. While not every SDN-capable device (node) needs a controller, you'll achieve better overall control and visibility using software-defined network controllers.
A good rule of thumb would be to have at least one extra software-defined networking controller than you need to run the network, and add an extra backup controller. Just like any other controller you have on the network -- be it wireless, voice or whatever -- it's commonsense to have a backup if possible.
When it comes to getting the right number of software-defined networking controllers, there even seems to be a common thread across different vendors' offerings. Cisco recommends using three of its Application Policy Infrastructure Controllers (APICs) that communicate with each other in a cluster for policy control and network management. Cisco's APICs are capable of controlling up to something on the order of a million devices.
For an NSX implementation, VMware recommends deploying an odd number of software-defined networking controllers. Obviously, you don't want just one, so it would have to be a minimum of three.
Three as the magic number also comes up for open source controllers OpenDaylight and ONOS. The reasoning is because of the work on the control plane, a lot of processing is going on, and you want to mitigate any problems related to a split brain resulting from the maintenance of two or three data sets that could overlap and create inconsistencies. Obviously, your network will continue to work if you lose connectivity with the controller because the data plane is separate, but you'll lose the visibility and policy control ability until the controller comes back online. Redundancy -- what a refreshing concept.
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