Recent advances in coherent detection and digital signal processing (DSP) technologies have stimulated the exploration of novel agile architectures for WDM core networks. Filterless optical networks are passive broadcast & select networks in which the need for agility is delivered by DSP-assisted tunable transceivers, and simple passive splitters/combiners are used for interconnecting the fiber links. The resulting network architecture is expected to bring about significant advantages, such as reduced installed first cost (IFC) of the network, ease of maintenance, and reconfigurability, as well as good resilience and multicast capabilities. In this paper, the filterless network design problem is defined for a wide area network (WAN), and a method based on genetic and Tabu search algorithms is proposed for designing long haul (LH) filterless networks based on the concept of “filterless islands”. Filterless network solutions are then proposed for a number of core network topologies, and the main characteristics of the solutions are presented. Our results show that filterless solutions can be found for different WAN topologies and confirm that the filterless networking concept is applicable to optical core networks.