The　complexity　of　data　networks　is　constantly　increasing,　as　does　the　occurrence　of　faults　and　misconfigurations.　Consequently,　network　operators　are　increasingly　concerned　with　the　negative　effects　that　misconfigurations　have　on　QoS　(Quality　of　Service)　and　the　difficulty　of　effective　troubleshooting.　This　paper　addresses　the　detection　of　Duplex　ity　Mismatch　(DM)　of　media　access　devices　in　Ethernet　networks.　From　the　broad　spectrum　of　sources　we　surveyed,　including　logging　data　from　real　networks,　it　appears　that　DM　is　surprisingly　common　and　very　severe.　We　show　how　DM　introduces　degenerative　traffic　anomalies　capable　of　drastically　reducing　flow　throughput.　We,　then,　propose　a　novel　detection　algorithm　based　on　end-to-end　active　probing.　Our　investigation　is　complemented　by　the　implementation　of　a　SW　prototype.　Extensive　experimental　evaluation　is　conducted　in　a　real-world　production　LAN.　The　achieved　results　are　encouraging　and　show　that　our　prototype　can　be　a　very　useful　tool.　Our　evaluation　attained　a　high　success　rate　of　misconfiguration　detection:　99.72%.　False　positive　and　false　negative　rates　are　extremely　contained:　0.00%　and　0.28%,　respectively.　Therefore,　our　prototype　appears　to　be　a　robust　and　reliable　detection　instrument　from　which　network　administrators　and　field　engineers　can　benefit.　©　2004　IEEE.