The application provides a multicast communications method and an aggregation switch, which can implement on-demand replication of multicast traffic. The method includes: an aggregation switch (AGG) acquires a multicast group address link state protocol data unit (GM LSP) packet sent by an access switch (ACC); the AGG compares multicast join information in the GM LSP packet with multicast join information stored in the AGG, to obtain a comparison result, and the AGG updates the multicast join information stored in the AGG according to the comparison result. The AGG generates a preset-protocol packet by using changed multicast join information in the comparison result according to a preset format, where the preset-protocol packet is an Internet Group Management Protocol (IGMP) packet or a Protocol Independent Multicast (PIM) packet. The AGG sends the preset-protocol packet to a first multicast router.