Before revision:  For both topologies , we develop fluid models and conduct a detailed local stability analysis in the small buffer regime, and obtain necessary and sufficient conditions for local stability.
After revision:  For both topologies , we develop fluid models in a regime wherein the number of flows is large, bandwidth-delay product is high, buffers are dimensioned small (independent of the bandwidth-delay product) and routers deploy a Drop-Tail queue policy. A detailed local stability analysis in the small buffer regime, and obtain necessary and sufficient conditions for local stability.