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High bandwidth and low latency switches are commercially available. Using these switches, it becomes possible to build a system area network to interconnect workstations and processor clusters together to provide a cost-effective parallel computing platform. A processor cluster may be a shared-memory multiprocessor or a mesh-connected multicomputer, etc. The interconnection topology on this kind of platform, called switch-based NOWP, is usually irregular. On such systems, multicast is an important collective communication operation. Two steps are involved in a multicast: (1) the source node sends the multicast message to the destinations which are connected to a switch directly or are the leader of a processor cluster, and (2) the leader node of each cluster sends the message to other destinations in the same cluster. In this paper, we propose two unicast-based multicast algorithms. Algorithm Multicast_1 performs those two steps sequentially; while Algorithm Multicast_2 overlaps them. Performance of the two algorithms will be evaluated and compared.