In multi-hop IEEE 802.15.4 networks with nodes operating in the beacon-enabled mode, the time and energy spent in the cluster-tree construction may be long and highly variable especially if there are no a priori restrictions on duty- cycle durations or on the channels used in the network. In this paper, we propose a Multi-Channel Beacon Train (MCBT) protocol in which coordinator nodes send trains of beacons on all channels at random instants during the inactive part of a superframe, so that nodes entering the network can quickly acquire the network parameters or routing metrics and join the network. The scheme drastically shortens the delay for topology construction and lowers the consumed energy.