Did you configure the STP priority? It is a best practice to set the Core switch to the lowest priority and for access switches you can use the default priority.   Also, verify that there is no other cable connected to the 3750x that could be causing a network loop.    I would recommend som Key points:   1. Use Rapid STP with command spanning-tree mode rapid  (default is STP ) 2.  Determine the best switch to be Root switch and set Bridge Priority (spanning-tree vlan x priority 0)   3. Adjust ports costs to Root switch (spanning -tree vlan x cost x)    STP decisions are based on the following sequence of four conditions: 1. Lowest root bridge ID 2. Lowest root path cost to root bridge 3. Lowest sender bridge ID 4. Lowest sender port ID ... View more