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STP Cost Paths Question

Hi everybody,

I am just an IT student and i am actually learning about STP protocol. At school they told us one thing that seems weird to me :

I precise that we decided to not consider the priority field (priority number + vlan id) in the Bridge ID section so that the Bridge ID is only composed of the Mac @ of the bridge.

When you have a bridge that is not directly connected to the root bridge but offers two paths that have the same cost to reach the root bridge, the bridge will select what ? the lowest path I guess (Lowest BID) but the teacher told us that only the root bridge election process used the lowest BID and that for the selection of one path between two paths with the same costs we must consider the highest BID (seems weird no ? Coz' with Cisco I learned that Lowest is best ....)

Thus i have an exam in 4 days and that i tried to join my teacher without success, i am asking the community.

Thanks to everybody that reads this post and therefore responds.

-- Ropza --


  • Hello 

    Here's the logic (algorithm) that makes the path selection when there is a tie between same cost and stuff like that. 

    1) first of all the highest priority will be given to the " priority field " and a mac address. [ KEEP IN MIND THAT IN STP WORLD LOWER IS BETTER ]. Incase of same priority, lowest MAC address will win.

    2) if tie between two same cost path occurs then  " Port ID " [e.g. 128 / (0/1) ] will be used to determine the path. 

    Ex.  (Least cost )--> if same then (compare BPDU ) ----> if same sending BID then ----> compare sending Port ID and there will not be same port id ever so somebody will win that election and will be the path to root bridge. 


    I hope this will help you. 



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