Power consumption and CO2 emission have become a significant concern over the last few years. Several recent studies have shown that servers and network equipment demand up to 45% of the energy consumption of data centers. Software-defined networking is a new networking paradigm that decouples the control and data functionalities; thus, it makes networks easily manageable and programmable. In software-defined networks (SDNs), the central controller has a global view of the network topology, traffic matrices and QoS requirements, which allows it to optimize the energy consumption of the network through energy-aware routing. In this paper, we investigate the impact of practical constraints, discreteness of link rates and limitation of flow rule space, on the performance of energy-aware routing schemes in SDN. The energy-aware routing problem is modelled as an integer linear program (ILP) with a discrete cost function. The problem is modelled in GAMS and solved by CPLEX under real network settings and practical constraints. Results show that considering these constraints is critical in order to exploit the energy-saving margin of SDNs.