Recent studies have shown that the energy consumption of wireless access networks is a threat to the sustainability of mobile cloud services. Consequently, energy efficient solutions are becoming crucial for both local and wireless access networks. In this paper, we propose a flow-based management framework to achieve energy efficiency in campus networks. We address the problem from the dynamic perspective, where users come and leave the system in unpredictable way. Specifically, we propose an online flow-based routing approach, that allows dynamic reconfiguration of existing flows as well as dynamic link rate adaptation, while taking into account users' demands and mobility. Our approach is compliant with the emerging Software Defined Networking (SDN) paradigm since it can be integrated as an application on top of an SDN controller. To achieve this, we first formulate the flow-based routing problem as an integer linear program (ILP). As this problem is known to be N P-hard, we then propose a simple yet efficient Ant Colony-based approach to solve the formulated ILP. Through extensive simulations, we show that our proposed approach is able to achieve significant gains in terms of energy consumption, compared to heuristic solutions and conventional routing solutions such as the Shortest Path (SP) routing, the Minimum link Residual Capacity routing metric (MRC) and the load balancing (LB) scheme. In particular, we show that the energy consumption can be reduced by up to 7%, 35%, 44% and 49% compared to Greedy-OFER, MRC, SP and LB, respectively, while ensuring the required Quality of Service (QoS).