One embodiment of the present invention provides a layer-3 forwarding device. The layer-3 forwarding device includes a processor and a computer-readable storage medium. The computer-readable storage medium stores instructions which when executed by the processor cause the processor to perform a method. The method comprises determining whether the layer-3 forwarding device is a leaf layer-3 forwarding device of a multicast distribution tree of a multicast group in a routed network based on a multicast topology discovery message from a root layer-3 forwarding device of the multicast distribution tree. If the layer-3 forwarding device is the leaf layer-3 forwarding device, the method comprises constructing a multicast topology report message. This multicast topology report message includes topology information of the multicast group in the routed network associated with the layer-3 forwarding device.