stp root port election

Answered Question
Sep 25th, 2013
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Dear All,


Please look at the attached picture. Switch 2 upper port is Gb1/1, which is blocked and the lower one is the Gb1/2, which is the root port. I dont't understand why not the upper port become to the root port... Because both of the mentioned port path cost is the same (16), and as I know in this situation the port with the lowest mac address win the election, which is the upper one (the upper port Gb1/1: 0060:4779:E401 while the Gb1/2: 0060:4779:E402)


Could you help me? Thank you


root port.jpg

Correct Answer by Lei Tian about 3 years 10 months ago

Hi,


SPT follows the following order to selecte root port. port number is the last.


1, lowest total paht cost

2, lowest neighboring switchID

3, lowest port priority

4, lowest port number


HTH,

Lei Tian

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Correct Answer
Lei Tian Wed, 09/25/2013 - 04:09
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  • Cisco Employee,

Hi,


SPT follows the following order to selecte root port. port number is the last.


1, lowest total paht cost

2, lowest neighboring switchID

3, lowest port priority

4, lowest port number


HTH,

Lei Tian

JohnTylerPearce Thu, 09/26/2013 - 08:24
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Just a quick follow up Mary,


on step 3 when you go to Port Priority as the poster above specified, there are two different settings with this.


Port Priority = 128.(interface number) usually


So for instance, if you have ports fa0/1 and fa0/10, they should be like this.


128.1 and 128.10


If you ever need to modify STP that far, you will need to understand those differences.

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