The Spanning Tree Protocol (STP) and its variants (Rapid STP, Multiple STP and Extended STP) are network layer (ISO-OSI Layer 3) protocols intended to provide a loop-free topology for bridged local area networks (LANs). STP is used almost exclusively in all local area networks. Since the proper functioning of the STP is vital to modern infrastructure and society, the dependability of STP implementations must be verified. STP Test suite is capable of testing STP implementations for security flaws and robustness problems.