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Traffic grooming, i.e. the aggregation of multiple traffic streams on one channel or wavelength, has often been considered in the context of reducing blocking and improving capacity utilization. More recently, traffic grooming has also been advocated in the context of energy-aware routing. In this paper we study energy-efficient path selection under the scheduled traffic model in IP-over-WDM optical networks. We show that there is indeed a strong relation between traffic grooming and energy efficiency, but also that it sometimes pays off not to groom. We propose an energy-aware routing algorithm that is based on traffic grooming, but which has the flexibility to deviate from it where needed. Our approach can be applied to networks with and without wavelength conversion. Simulations show that our energy-aware routing algorithm both brings significant energy savings as well as a lower blocking probability in …