I have a simple topology made of a couple of Cat6500 switches (native IOS) connected between them and to a Cat3750 (stack of two). I have MST running. When adding a new Vlan it gets propagated to all the switches via VTP, when I assign it to a MST instance on the first switch (on the other switches it is still assigned to instance 0) a spanning tree loop occurs, connectivity to the switch is lost and the only way to recover from this situation is to manually break the loop by disconnecting a cable. After breaking the loop and assigning the new vlan to the correct MST instance the cable is put back in place and everything works fine. Any idea why this would happen?
Many thanks in advance.