We consider the design of a hybrid opto-electronic network with grooming. We propose a model in which we take into account the fact that grooming traffic requires o-e-o conversion and processing by the electronic switch. This in turn means that a new switch has to be installed whenever its processing capacity is exhausted so that grooming cost exhibits a very large modularity, an element that is missing from all current models. We show on small network examples that this modularity has a very strong impact on the structure of the dimensioning and routing of the demands.