| Upgrading a legacy network to a Software Defined Network (SDN) in stages, and minimizing the energy consumption of a network are now of great interests to operators. To this end, this paper addresses a novel problem: minimize the energy consumption of a network by upgrading switches over multiple stages subject to the available monetary budget at each stage. Our problem considers (i) bundled links that can be powered-off individually, (ii) decreasing upgrade cost and increasing traffic demands over multiple stages, and (iii) rerouting demands to an alternative path with delay that is within a predefined limit and that the link load on the path is no larger than a given maximum utilization. We formulate the problem as an Integer Linear Program (ILP) and propose a greedy heuristic called Green Multi-stage Switch Upgrade (GMSU). Experiment results on five actual network topologies show that (i) our approaches reduce energy consumption by up to 73.98\%, (ii) rerouting traffic demand via longer paths improves energy saving by up to 9.6\%, (iii) GMSU produces results that are at most 4.83\% from the optimal result, and (iv) increasing the budget as well as number of stages affect the total energy saved and number of upgraded switches. |