Advanced Spanning Tree

root primary - sets the priority to:
if ROOT > 24576 - sets to 24576 (priority 24576 sys-id-ext 12)
if ROOT =< 24576 - sets to 4096
root secondary - sets the priority to 28762

GREAT COMMAND:
#show spanning-tree bridge <- See the MAC address of the Switch
#show version | i Base 

Cat-1#show spanning-tree vlan 12
VLAN0012
  Spanning tree enabled protocol ieee
  Root ID    Priority    24588              <--- ABOUT THE ROOT BRIDGE, 24588 = 32768 + 12 (vlan 12) - 8192
             Address     ec44.768a.6d80
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    24588  (priority 24576 sys-id-ext 12)    <--- ABOUT THIS SWITCH (LOCAL Bridge)
             Address     ec44.768a.6d80 <-- ON ROOT BridgeID and RootID have the same MAC
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type           <--- ABOUT INTERFACES IN THIS VLAN 
------------------- ---- --- --------- -------- ------
Gi3/0/19            Desg FWD 4         128.127  P2p     <--- COST IS 4 CAUSE THIS IS GigabitEthernet Port 
Gi3/0/20            Desg FWD 4         128.128  P2p          (on FastEth is would be 19)

Great command to check the ROOT>
#show spanning-tree root

                                        Root    Hello Max Fwd
Vlan                   Root ID          Cost    Time  Age Dly  Root Port
---------------- -------------------- --------- ----- --- ---  ------------
VLAN0001         32769 aabb.cc00.0600       200    2   20  15  Et2/2
VLAN0100         24676 aabb.cc00.0600       200    2   20  15  Et2/2
VLAN0200         24776 aabb.cc00.0700       100    2   20  15  Et2/2
VLAN0300         24876 aabb.cc00.0800       100    2   20  15  Et3/1
VLAN0400         24976 aabb.cc00.0900         0    2   20  15 <--- COST TO ROOT IS 0, SO I'm the ROOT!!!

BEST PRACTICE:
Change the COST on the interface level to change the PATH
Change the PORT PRIORITY to influence ONLY the NEIGHBORING SWITCH