Application-layer multicast (ALM) can easily address most of the problems of IP-based multicast such as forwarding state scalability, full router dependence and so on. The topology-aware approach of ALM is more attractive because it exploits underlying network topology data to construct multicast overlay networks. In this paper, a more efficient mechanism of overlay construction called Subnet Topology-Aware Grouping (STAG) is introduced. Under this mechanism, new node who wants to join the multicast tree will broadcast a JOIN message in the subnet firstly or at the same time when it sends a JOIN message to source member. It can save the time used to discover topology information and execute the path matching algorithm if there are some multicast members in the same subnet. Simulation results show that new nodes of STAG can acquire multicast service more quickly with similar or less overhead when the group size is large or the percentage of subnet nodes is high.
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