Check all your switches and make sure that the spanning tree mode is the same. BOS-2960PDU-SW1#show spanning-tree summary Switch is in rapid-pvst mode Root bridge for: none EtherChannel misconfig guard is enabled Extended system ID is enabled Portfast Default is disabled PortFast BPDU Guard Default is disabled Portfast BPDU Filter Default is disabled Loopguard Default is disabled UplinkFast is disabled BackboneFast is disabled Configured Pathcost method used is short Name Blocking Listening Learning Forwarding STP Active ---------------------- -------- --------- -------- ---------- ---------- VLAN0010 0 0 0 5 5 VLAN0033 0 0 0 1 1 VLAN0034 0 0 0 1 1 make sure that all the vlans are being properly sent out the trunk interfaces BOS-2960PDU-SW1#sh int trunk Port Mode Encapsulation Status Native vlan Po1 on 802.1q trunking 900 Po48 on 802.1q trunking 900 Port Vlans allowed on trunk Po1 10,64-71 Po48 10,33-40,50,64-71,1001 Port Vlans allowed and active in management domain Po1 10,64-71 Po48 10,33-40,50,64-71,1001 Port Vlans in spanning tree forwarding state and not pruned Po1 10,64-71 Po48 10,33-40,50,64-71,1001 BOS-2960PDU-SW1# |