A communication system includes: a first base station that operates as a donor for integrated access and backhaul, the first base station including a central unit (IAB-donor CU) and a distributed unit (IAB-donor DU); and one or more second base stations that operate as nodes of the integrated access and backhaul (IAB-nodes), and in a case where multicast transmission of data is performed from the first base station to a communication terminal (UE), the central unit performs a configuration for the multicast transmission on the distributed unit and the one or more second base stations.