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As Internet traffic is estimated to substantially grow in the near future, one of the most challenging issues will concern the reduction of its power requirement. For the core/transport section of the Internet, various network architectures can be considered, typically multi-layer architectures composed of an optical wavelength division multiplexing (WDM) transport layer under the classical electronic IP layer. Thus, we consider in this paper four architectures: basic IP over WDM with no optical switching, IP over synchronous digital hierarchy (SDH) over WDM, and IP over WDM with transparent or translucent switching. Energy efficiency in these architectures is expected to be enabled by optical switching technologies, mainly due to the significant reduction in the number of required optical/electronic/optical conversions. However, since optical signals are subject to relevant physical layer impairments when traversing core network devices, signal regeneration is often required, which has to be accomplished either at the electronic layer by routers/digital cross connects (DXCs) or directly at the optical layer through 3R-regenerators. Therefore, it is important to know which network architecture may provide the highest energy efficiency. In this paper we perform a comprehensive comparison between the four above-mentioned optical core network architectures, by performing energy assessment of the devices used at the transport layer of a telecom network and by developing an integer linear programming formulation for an energy-minimized and impairment-aware design of each of the considered architectures. We find that optical technology can enable power savings up to 60% with respect to classical IP-over-WDM architectures with no optical switching, but the best “optical” architecture is not univocal, and it is influenced by some crucial network parameters (connectivity, geographical extension, etc.).