1. A method comprising: generating network topology structure data describing at least part of a topology of a first network, the first network having a plurality of first peer nodes providing peering links to a plurality of second peer nodes in a second network, the plurality of first peer nodes of the first network being connected to corresponding ones of the plurality of second peer nodes of the second network; generating demand structure data describing at least some traffic demands relating to a source in the first network and a destination in the second network, wherein there are a plurality of network links between the first network and the second network, the plurality of network links being peering links; 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, the calculation of routing of traffic for the at least one change scenario including calculating how much traffic will be routed through each of the plurality of network links in the event of one or more of the network links being changed; 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, the change data being determined from the calculation of 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.