Solved: stp root port election

Mary Poppins · ‎09-25-2013

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

Lei Tian · ‎09-25-2013

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

View solution in original post

Lei Tian · ‎09-25-2013

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

Mary Poppins · ‎09-25-2013

Oh, I see,

Thank you very much

milan.kulik · ‎09-25-2013

Hi,

nicely explained here:

http://ciscoiseasy.blogspot.cz/2010/10/lesson-20-spanning-tree-protocol.html

Best regards,

Milan

Mary Poppins · ‎09-26-2013

Thank you very much, very useful!

JohnTylerPearce · ‎09-26-2013

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.