An optimum configuration of resources in a network function virtualisation data network is identified by assembling candidate configurations of resources (243), each configuration being an arrangement of the resources into clusters selected such that each cluster provides one or more required services, (212, 213) and assessing the candidate configurations (step 400) to identify an optimum configuration, the assessment of each configuration including measurement of latency (195) in physical links between the resources and, for each candidate configuration, determination of the total latency between the resources within each cluster of the configuration, for a predicted level and pattern of traffic associated with the required service to be operated by each cluster.