=====WHY=====
Frame Loop Prevention Mechanism. Achieved by blocking all but one **''ROOT''** port to a **''ROOT''** switch. Redundant links are then blocked to create a loop-free environment.

=====CONCEPT=====
  * It does this with Each Switch broadcasting a **''HELLO BPDU''**, made up of that SW's **''Priority + MAC''**. Default Priority is 32768. The SW with the **''lowest BPDU''** (superior) is elected the ROOT BRIDGE. 
  * Each SW will then determine a **''ROOT PORT''**, one per SW. A **''DESIGNATED''** port is then elected on each segment. 
  * There can only be **one DESIGNATED port per SEGMENT/LINK. **
  * Any port that is **not** **''DESIGNATED''** or a **''ROOT''** port, is then set to a **''BLOCKING''** state.
Note that once a ROOT Bridge is elected, all other bridges will only forward HELLO BPDU's **originating** from the **''ROOT BRIDGE''**. The Root cost is incremented as the **''HELLO BPDU''** is **received (COST=$COST+$PORT_COST)**. The Root Cost **starts off @ 0** from the **''ROOT''**. Each subsequent Bridge tacks on it's **COST TO THE ROOT BRIDGE** as it is **received**.


==TO DETERMINE A DESIGNATED PORT:==
  * --Is my cost lower to the ROOT BRIDGE?
  * --COST IS SAME: Is my my Bridge ID lower?
  * --if NO to either, PORT WILL BE BLOCKED!

=====COMMANDS:=====
====Quickly setting a PRIMARY and SECONDARY ROOT:====
<code>SW1(config)#spanning-tree vlan 1 root primary 
SW1(config)#spanning-tree vlan 1 root secondary</code>

====Manually set Bridge Priority for Root Election:====
<code>SW2(config)#spanning-tree vlan 1 priority ?
  <0-61440>  bridge priority in increments of 4096
  
SW2(config)#spanning-tree vlan 1 priority 4096

                                                   Hello  Max  Fwd
Vlan                         Bridge ID              Time  Age  Dly  Protocol
---------------- --------------------------------- -----  ---  ---  --------
VLAN0001         4097(4096,      1) 0019.e86a.2280    2    20   15  ieee      <-- LOWER PRIORITY = ROOT BRIDGE!
VLAN0002         32770(32768,    2) 0019.e86a.2280    2    20   15  ieee</code>

====Manually Define Port Cost for Root Port Election:====
<code>SW2(config)#int fa0/1
SW2(config-if)#spanning-tree vlan 2 cost 1
SW2(config-if)#

SW2(config-if)#do sh spann int fa0/1

Vlan                Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
VLAN0001            Desg FWD 19        128.1    P2p
VLAN0002            Root FWD 1         128.1    P2p       <-- NOTICE THE COST! LOWER COST = ROOT PORT!</code>