STP port-priority problem

ok I've set up this lab for practice.

•I've configured VLANs 50, 60, 70 and switchport access them into the PCs
•Configured Inter-VLAN
•Configured STP, for VLANs 50,60,70 the root bridge is S6 so for VLAN 1 by default the root bridge is at S7 (lowest MAC address)

We all know that STP uses port-priority, and on a redundant connection it will block the port that has the high value (ex. Fa 0/1 128.1, Fa 0/2 128.2), without doing anything by default it will set Fa0/1 as the root port and Fa 0/2 as the Alternate port. By configuring port-priority on the interface (increment of 16) we can choose which port will be the Root Port and the Alternate Port. Have I said this correctly?

NOW.. I have a problem on S8. S8 has the higher value of mac address compare to S7 so basically on S8 it will have 3 ports to be set as Alternate ports. The ports that have been blocked by the S8 are Fa 0/3 & Fa 0/4 (because these ports connects to S7) and the last port to be blocked is Fa 0/1 (I have not done anything besides configuring root primary at S6).

THE PROBLEM:
Regarding the S8, would it by default should blocked Fa 0/2 instead of Fa 0/1 because of the Port-priority? So, after that I try to resolved the problem and set the port-priority of Fa 0/1 to 96 (that is lower than the port priority of Fa 0/2 which have 128.1) now i'm expecting a change or port role. I save the configuration and reload the switch. To my disappointment nothing changes! what seems to be wrong?

P.S: I can't upload the configs, the attachment button won't accept .txt files and .doc. is there other way around to post the configs. thank you!

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Comments

Zartanasaurus

STP uses received port priority, not the priority that is configured on its own interface. You can change priority all day on S8 on its connections to S6 and it won't matter. Looks like you have S6 0/2 plugged into a lower port # on S6 than S0/1 is plugged into.

If you want to affect the STP selection process with port priority, you do it on the OTHER switch.

mapletune

Agree 100% with above.

The reason why the important port is the "other" switch's port, is because priority is calculated from received hello bpdu.

A switch can receive many hello bpdus right? it'll look at those and calculate which is optimal to get to "root" or "desg" etc. It's own priority doesn't matter until it sends it's own hello bpdus out to other switches, which it adds its own priority. so on so forth.

CaptainL

Zartanasaurus wrote: »

STP uses received port priority, not the priority that is configured on its own interface. You can change priority all day on S8 on its connections to S6 and it won't matter. Looks like you have S6 0/2 plugged into a lower port # on S6 than S0/1 is plugged into.

If you want to affect the STP selection process with port priority, you do it on the OTHER switch.

thank you!! now I understand it

mapletune wrote: »

Agree 100% with above.

The reason why the important port is the "other" switch's port, is because priority is calculated from received hello bpdu.

A switch can receive many hello bpdus right? it'll look at those and calculate which is optimal to get to "root" or "desg" etc. It's own priority doesn't matter until it sends it's own hello bpdus out to other switches, which it adds its own priority. so on so forth.

thank you for expounding