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--- icnd2:spanning-tree [2024/04/24 17:13]
114.119.141.71 old revision restored (2018/01/13 10:48)
+++ icnd2:spanning-tree [2024/05/15 02:53] (current)
114.119.138.183 old revision restored (2024/02/13 19:32)
@@ Line 1: / Line 1: @@
 =====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.
+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 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.
+  * 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.
+  * Each SW will then determine a **''ROOT PORT''**, one per SW. A **''DESIGNATED''** port is then elected on each segment.
-  * Any port that is not DESIGNATED or a ROOT port, is then set to a BLOCKING state.
+  * 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:==
@@ Line 13: / Line 16: @@
 =====COMMANDS:=====
-==Quickly setting a PRIMARY and SECONDARY ROOT:==
+====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:==
+====Manually set Bridge Priority for Root Election:====
 <code>SW2(config)#spanning-tree vlan 1 priority ?
   <0-61440>  bridge priority in increments of 4096
@@ Line 25: / Line 29: @@
 Vlan                         Bridge ID              Time  Age  Dly  Protocol
 ---------------- --------------------------------- -----  ---  ---  --------
-VLAN0001         4097(4096,      1) 0019.e86a.2280    2    20   15  ieee
+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
+VLAN0002         32770(32768,    2) 0019.e86a.2280    2    20   15  ieee</code>
-  </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>

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