A mechanism for performing routing in a communication system including a radio access network and a plurality of core networks connected to the radio access network. In order to accomplish a mechanism whereby the routing of a registration request to a serving core network, and also the rejection of a registration request, can be carried out in a controlled manner, a core network is first selected for a registration request received. The registration request is then forwarded to the selected core network and, in response to at least one predetermined criterion being fulfilled, the selected core network is notified that the registration request is to be served by that core network, i.e. that the registration request cannot be rerouted to another core network.