Abstract: The main achievements were introduced in physical topology discovery for Ethernet and the defaults of those methods were pointed out,then a new algorithm was proposed based on the intersection of multi-subnet．The algorithm uses the Minimum Requirements of Address Forwarding Table(AFT) around the intersection of multi-subnet to reason and establish the connection between switchers.And combining a typical multi-subnet topology,the algorithm was derived．Theoretical analysis proves that our method correctly infers the network topology with the incomplete AFTs,and has low communication and computational overheads,in which other methods fail．The algorithm is appropriate for discovering the physical topology of large,heterogeneous Ethernet that may include multiple subnets as well as uncooperative network elements,like hubs.

Key words: Physical topology,Connecting tree of subnet,Intersection node,Minimum requirements of address forwar-ding table