Abrer_Wiki

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(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!