Due to high installation and maintenance costs of optical WDM (wavelength division multiplexing) networks will be deployed with some optimization criteria in mind. Here we study the problem of designing the logical topology and routing of traffic, with the objective to minimize congestion while constraining the average delay seen by a source-destination pair and number of wavelengths used, for a given network physical topology and traffic pattern. The adopted method divides the problem into two parts: the logical topology design, implemented using heuristic algorithms, and traffic routing, formulated as a linear programming problem. We suggest that an aggregated formulation should be used for the traffic problem and that post-optimal analysis can be used to keep the traffic optimised on the network