In this paper we investigate the options of a network operator faced with the requirement of reducing its carbon footprint, expressed in terms of a global energy cap. First, we propose two ways to meet the energy limitations: by efficiently managing the energy consumed by the legacy networks or by installing additional capacity to the initial topology. We show the power savings that can be obtained in both cases as well as the incurred costs. Then, we identify the initial composition of the network and the available technology in the upgrade phase as the factors that have the most influence on the ability of a network to meet the energy caps. Finally, we show the intrinsic unfairness of the energy caps, which are imposed to all the networks without taking into account the differences among them. Therefore, we highlight the fundamental role of carbon markets and emission trading systems in guaranteeing a measure of fairness between the operators.