STP port-priority

[danielsffs]

Hello everyone,

I'm configuring 3 switches and I want to control which port will become the Alternating/Blocking port. I have configured the port that that should be blocked PO3 on S2 as port priority 0 and the port PO3 on S3 as port priority 240. For some reason PO3 on S2 will remain in blocking state. 

I added the topology with the current and desired state. I added the output of the command Show spanning-tree stp 0 aswell.

Thanks in advance.

Regards,

Daniel

[balaji.bandi]

If S1 is Root bridge for all VLANs,  if i understand correctly that is expected behaviour.

Some concept from my notes :

If you want to affect how to the local switch elects the root port change the cost on the links. Cost is cumulative throughout the STP domain. The higher cost is the less preferred.

If you want to affect how downstream switch elects its root port change the priority. This is only local significant between the two directly connected switches. Highest priority is less preferred.

[danielsffs]

Okey, I understand. But how on the link between S2 en S3 how do the switches negotiate which interface will go into blocking state and which will remain in designated state? Because now the interface on S3 is in blocking state but why did the STP choose the interface on S3 and not the interface on S2?

Thanks for your fast reply!

Regards,

Daniel

[balaji.bandi]

Root bridge election based in the criteria (explained well here)

https://www.firewall.cx/networking-topics/protocols/spanning-tree-protocol/1054-spanning-tree-protocol-root-bridge-election.html

Why only certain device  - we required all device show run ( along with show spanning brief)

[Giuseppe Larosa]

Hello @danielsffs ,

lowest port priority is preferred exactly as lowest MAC address is preferred.

You should revert your STP priority configuration if you want the Designated Port to be on S2 side.

What you see is normal and expected behaviuor for the way any STP flavor including MST work.

Hope to help

Giuseppe

[Aladdin0z]

hello @danielsffs 

i am quoting from you "lowest port priority is preferred exactly as lowest MAC address is preferred. "

You mean that Mac addresses and port priority are checked at the same time ? 

i thought the mac is checked first 

[Georg Pauwen]

Hello,

you are running MST and port channels. How did you configure the channels ? If possible, post the full configs of all three switches, maybe we can spot something...

[danielsffs]

Hello Georg,

The config as requested. 

[danielsffs]

Okey I figured it out btw. When changing the spanning-tree priority on the switches I can control which port will become a alternating port and which port wil become a designated port. 

I gave S3 the command: spanning-tree mst 0 priority  4096 

Now PO2 on S3 is in an designated port.