This letter proposes an ant colony optimization (ACO) based energy saving routing, referred to as A-ESR, for energy efficient networks. The proposed A-ESR algorithm firstly re-formulates the energy-consumption minimized network (EMN) problem, which is NP-complete, into a simpler one by using the concept of traffic centrality. After that, it solves the re-formulated problem by 1) letting the flow to autonomously be aggregated on some specific heavy-loaded links and 2) switching off the other light-loaded links. Simulation results show that the A-ESR algorithm can get better performance than previous works in terms of energy efficiency.