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.

• 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.

• –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!

• 10Mb link: 100
• 100Mb link: 19
• 1Gb Link: 4

SW1 <> SW2 (Fa0/11→ Fa0/11, Fa0/12 → Fa0/12)

The port that recieves the lowest BPDU is elected the root port. Considering the above topology, SW2 will receive the same BPDU from SW1 on BOTH PORTS. With the BPDU the same, it ties. A tie-breakier is needed to determine the ROOT PORT.

stuff

SW1(config)#spanning-tree vlan 1 root primary 
SW1(config)#spanning-tree vlan 1 root secondary

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

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!