1. A method of managing a transition for a class of data between first and second topologies in a data communications network comprising a plurality of nodes and supporting multiple topologies, the method comprising the steps, performed at a participating node of: defining a transition topology that is used temporarily during a routing transition from the first network topology to the second network topology; marking data in said class as assigned to the transition topology; wherein the marking constrains the data to travel in the transition topology assigned to the marking; forwarding marked data in said class only among nodes within said transition topology; ceasing marking data when all participating nodes in the network have implemented the marking step; and subsequently forwarding data in said class via the second topology.