Software Defined Networking (SDN) has the promise of flexible routing, traffic management and service provisioning in communication networks. To allow SDN based networks scale in size, the control architecture needs to be distributed, which in turn requires the introduction of controller to controller communication. This is needed to ensure that the distributed controllers have the same understanding about the underlaying network and can make consistent local decisions. In this paper we evaluate the volume of the emerging control traffic, considering a distributed controller architecture based on ONOS and OpenFlow. We show that the control traffic increases drastically with the number of controllers, as well as with the size of the underlaying network. We evaluate topologies forming regular and random graphs, and conclude that the type of the topology influences the traffic volume significantly, while the network density has less significant effect. We show that the control traffic is significant even if the number of controllers is selected such that the control traffic is minimized, and we argue that further optimization of ONOS is needed to trade off control traffic load and consistency in the network views.