Shortest-path spanning trees are created outward from each portal on a Rbridge network with nodes on the network being assigned to the spanning trees. For those nodes equidistant to two portals, an election process is used to determine which tree the node should join. To enable routes associated with nodes on the Rbridge network to be distinguished from routes external to the Rbridge network, messages may be transmitted between the portals. The messages enable portals to identify routing updates received from the external network as containing internal routes, so that traffic that is required to pass between nodes on different trees may pass over the Rbridge network rather than over the external network.