A method of forwarding a multicast data by a system being an extended bridge constructed by a controlling bridge and a plurality of port extenders, wherein the controlling bridge is connected to an upstream port of each port extender via a cascade port, the method comprising: receiving, by the controlling bridge, a multicast data packet; determining, by the controlling bridge, a multicast E-Channel Identification, ECID, and a cascade port associated with a multicast group of the multicast data packet on the controlling bridge, wherein the multicast ECID directs to egress ports in different Virtual Local Area Networks, VLANs, of the multicast group on a port extender connected with the cascade port; adding, by the controlling bridge, an Extension VLAN Tag, ETAG, having the multicast ECID to the multicast data packet; and transmitting, by the controlling bridge, the multicast data packet with the ETAG having the multicast ECID via the cascade port, receiving, by the port extender, the multicast data packet with the ETAG having the multicast ECID via the upstream port; searching, by the port extender, for multicast duplication information matching the multicast ECID, wherein the multicast duplication information contains a port identification and a VLAN identification of each egress port of a multicast group corresponding to the multicast data packet; and duplicating, by the port extender, the multicast data packet for each egress port based on the multicast duplication information, removing the ETAG of each copy of the multicast data packet, adding the VLAN identification of each egress port to each copy of the multicast data packet, and transmitting the multicast data packet containing the VLAN identification via each egress port. receiving, by the controlling bridge, a multicast data packet; determining, by the controlling bridge, a multicast E-Channel Identification, ECID, and a cascade port associated with a multicast group of the multicast data packet on the controlling bridge, wherein the multicast ECID directs to egress ports in different Virtual Local Area Networks, VLANs, of the multicast group on a port extender connected with the cascade port; adding, by the controlling bridge, an Extension VLAN Tag, ETAG, having the multicast ECID to the multicast data packet; and transmitting, by the controlling bridge, the multicast data packet with the ETAG having the multicast ECID via the cascade port, receiving, by the port extender, the multicast data packet with the ETAG having the multicast ECID via the upstream port; searching, by the port extender, for multicast duplication information matching the multicast ECID, wherein the multicast duplication information contains a port identification and a VLAN identification of each egress port of a multicast group corresponding to the multicast data packet; and duplicating, by the port extender, the multicast data packet for each egress port based on the multicast duplication information, removing the ETAG of each copy of the multicast data packet, adding the VLAN identification of each egress port to each copy of the multicast data packet, and transmitting the multicast data packet containing the VLAN identification via each egress port.