Multi-domain multi-controller software-defined networks have an inevitable potential to improve the scalability and reliability of massive data center networks because the single controller cannot match the rapid expansion of cloud computing. The inconsistency among controllers can significantly degrade the network performance, because of that the data center network in software-defined networks has physically distributed control plane but operates with logically centralized plane. Existing synchronization among multi-controllers is based on the periodic synchronization (PS) that is stiffly triggered by time. However, the PS neglects potential inconsistency and further produces much packet loss and poor load balance in the whole network. Longer synchronization period increases synchronization overhead and decreases network performance. To keep the load consistency among controllers, we propose an active synchronization algorithm that is agilely triggered by event, ie, synchronization is triggered when the load of specific server exceeds the average load of its domain. With such an event triggered mechanism, we cannot only eliminate the correlation between synchronization and time but also avoid forwarding loop in the global network. Simulation results show that compared to existing PS-based schemes, our active synchronization algorithm can achieve better load balance with less synchronization overhead and lower packet loss rate. [ABSTRACT FROM AUTHOR]