In Software Defined Networks (SDN), the switches are statically assigned to controllers, which cause load imbalance among controllers. Among other problems, congestion, delay in SDN control messages, and controller overload are usually risen, especially in high traffic networks like data centers. In this work, we consider a distributed SDN controlling switches running OpenFlow protocol. We propose an add-on algorithm implemented on the controllers, whose aim is to dynamically assign switches to controllers whenever overload or failure is observed. Consequently, this will preserve the control plane and improve controllers performance. Simulations showed that such algorithm improves performance and reliability, and it is a must in a distributed SDN.