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what is the purpose of an sdn controller
An SDN controller is an application in a software defined networking architecture that manages flow control for improved network management and application performance.
What does the SDN controller run on
The SDN controller runs on a server
How do SDN controllers direct traffic
SDN controllers direct traffic according to forwarding policies that a network operator puts in place, thereby minimizing manual configurations for individual network devices.

Question: How do controllers communicate
Self-contained rewrite: How do SDN controllers communicate