The ever-increasing Internet Protocol (IP) traffic volume has finally brought to light the high inefficiency of current wavelength-routed over rigid-grid optical networks in matching the client layer requirements. Such an issue results in the deployment of large-size, expensive, and power-consuming IP/Multi-Protocol Label Switching (MPLS) layers to perform the required grooming/aggregation functionality. To deal with this problem, the emerging flexgrid technology, allowing for reduced-size frequency grids (usually referred to as frequency slots), has recently attracted much attention among network operators, component and equipment suppliers, and the research community. In this paper, we tackle the multilayer IP/MPLS-over-flexgrid optimization problem. To this end, an integer linear programing formulation and a greedy randomized adaptive search procedure (GRASP) metaheuristic are provided. Using GRASP, we analyze the cost implications that a set of frequency slot widths have on the capital expenditure investments required to deploy such a multilayer network. For the sake of a compelling analysis, exhaustive numerical experiments are carried out considering a set of realistic network topologies, network equipment costs, and traffic instances. Results show that investments in optical equipment capable of operating under slot widths of 12.5 GHz, or even 25 GHz, are more appropriate, given the expected traffic evolution.
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