In a Shortest Path Bridge (SPB) network comprising a plurality of backbone edge bridges (BEBs), a gateway controller of the SPB network, establishes a link using Multicast Source Discovery Protocol (MSDP) with an external network domain; discovers via the link a source of a multicast stream originating outside of the SPB network; and determines a subset of the plurality of BEBs that are able to receive the multicast stream from the source. Once that occurs, the gateway controller selects a one of the subset of the plurality of BEBs to be a sole gateway BEB for the multicast stream; and then transmits to each of the subset of the plurality of BEBs an indication of the sole gateway BEB selected for the multicast stream.