1. A network management system for management of a traffic network comprising a wired network backbone and a wireless network backbone comprising a plurality of relay nodes comprising wireless routers, some of which acting as gateways to the wired backbone network and some of which acting as relay bridge routers supporting access of a number of access networks, said management system comprising a number of management stations arranged in a distributed manner and adapted to handle sub-networks of said traffic network, wherein the management system comprises a logical multilevel hierarchical overlay network, with a top or central manager, based on the infrastructure of the traffic network and adapted to use the relay nodes as management stations, the management system comprises a grouping processor that performs a grouping process by arranging network nodes, comprising the relay nodes, in groups or clusters, each with a cluster head, on different hierarchical levels, and further comprising a grouping controller that controls the grouping and allocation of cluster heads based on a number of grouping constraints given by one or more criteria to be fulfilled at each hierarchical level, a criteria calculator that calculates the criteria based on given metrics, and a modelling and verification processor that models the constraints and verifies fulfilment of said constraints at each hierarchical level.