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Spanning Tree Protocol (Root, Designated, Alternate Port)

Hello everyone, in previous article I have discuss about Spanning Tree Root Bridge.  And in this article I’ll continue to discuss about Spanning Tree Protocol for Root Port, Designated port and Alternate port.

Root Port

After the root bridge has been determined, the STA algoritm is used to select the root port. Every non root bridge will select one root port and root port is the port closest to the Root Bridge in terms of overall cost to the root bridge.  In other words port that have lowest cost to root bridge will become a root port.


Designated Port

Every port segment are have one designated port. The designated port is a port on the switch that has internal root path cost. in other words designated port has best path to receive traffic leading to the root bridge. Port that is not designated or root port will become alternate port.  Switch port on the root bridge will be designated port and also port that connected to the end user will become designated port.


Alternate Port

Alternate port is a port that non root port or designated port. This port state will be a backup port and it doen’t forward any regular traffic to prevent loop.


Election root port from multiple equal cost paths

When the switch has multiple links or multiple equal cost paths leading to the root bridge, switch will be determine port with the following criteria :

  • Lowest sender BID
  • Lowest sender port prioriry
  • Lowest sender Port ID


As the following example bellow, There are four switch that connected each other and they hvae sam priority. See that SW1 become Root Bridge because it has a lowest mac address between others. Then all ports on SW1 will become Designated Port. SW2 and SW3 interface G0/2 will become Root port because they are direct connect to the root bridge and other port will be Designated port.

Let’s focus on SW4. This switch will send traffic to the Root bridge between through SW2 or SW3.  Which are best cost path ? yes Best path on SW4 leading to Root Bridge is G0/2 or SW3. Why is that ? Let’s compare between SW2 and SW3 first look at the cost. If we calculate total cost for SW4 leading to Root Bridge are same 8.  So let’s check next criteria and we got information that on the SW3 has the lowest mac address than SW2 so best path will select from SW3.

See on the following example. There are two switch that connected with multiple links. SW1 are Root Bridge so all of interfaces is Designated Port.  Default port priority on interface is 128 so all interface have same priority. But what’s criteria to determine Alaternate port ? .  See on Root Bridge for port ID, interface G0/1 is lowest than G0/2 so on SW2 interface G0/1 that connected to G0/1 Root Bridge will become Root port while SW2 G0/2 that connect to G0/2 Root Bridge will become Alternate port


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