In the situation that data switching is incorporated in a wide area telecommunications network it is advantageous to use a local area network protocol such as ethernet which allows network resources to be shared. However, this introduces a problem with respect to loops in the network and the possibility of infinite packet looping. If the known spanning tree algorithm is used to prevent infinite packet looping this is problematic because the known algorithm takes a relatively long time to take effect. An algorithm is described which is based on assumptions about the topology of the communications network and which is simpler and faster.