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In the present paper, we propose a practical and feasible method to construct VLAN-based fat tree network that provides wide bisection bandwidth on a tree network constructed with cheap Layer-2 Gigabit Ethernet switches for high-performance PC clusters. The proposed method provides a complete flat IP address space with multiple upper-level links and switches to configure a fat tree based on simple routing by IEEE 802.1q tagged VLAN. We modified the pseudo device driver for tagged VLAN on the Linux operating system so as to implement an enhanced control feature in IEEE 802.1q frames in order to build a large-scale fat tree. As a result of preliminary performance evaluation on a small cluster, we confirmed that the proposed system works correctly without increasing the overhead and an ordinary MPI library can be applied without modification to provide wide bisection bandwidth on the system.