1. A method of communicating predicted network behavior, the method including: generating network topology structure data describing at least part of a topology of a first network; generating demand structure data describing at least some traffic demands relating to a source in the first network and a destination in a second network, wherein there are a plurality of network links between the first network and the second network; generating traffic routing change data describing at least one change scenario which would require a change of traffic routing; using the network topology structure data, the demand structure data and the traffic routing change data to calculate a routing of traffic through each of the plurality of network links between the first network and the second network for the at least one change scenario; and transmitting change data to the second network, the change data describing network traffic movement through the plurality of network links in the at least one change scenario.