A single step routing and wavelength assignment method and system for automated provisioning of services on DWDM networks is presented. This novel single step solution automates design and assignment of services in DWDM networks. For an automated provisioning platform that can handle the routing and wavelength assignment in a single step, the solution avoids reconfiguration of existing services. It also takes into consideration practical aspects of DWDM transponder availability at termination sites and regeneration sites along the selected route. The methodology includes iterative computation of common channel sets to avoid multiple shortest path computations for each of the wavelengths.