Physical network topology discovery is a key issue for network management and planning, performance forecasting, network simulation and security; and how to discover a physical network topology based on address forwarding table (AFT) is a hot topic in current studies. This paper defines minimal constrains on AFT Tables for a switched area of a single subnet or multiple subnets to deducing its physical topology, and proves the completeness of the basic reasoning rule (BRR) proposed in the previous work. Furthermore, the paper analyzes the NP hard problem of AFT based methods, thoroughly discusses all kinds of possible situations deduced by BRR for any local network, and further investigates the limits solely based on AFT topology discovery. This work provides very important theoretical guidance in physical topology discovery based on AFT, and at the same time lays a solid theoretical foundation for new topology discovery methods.