STP - Switch Port Identifier

[RaphaelL]

Hi ,

I have a simple setup which includes a switch stack of 6 MS-350 and this stack is the root bridge.

SW1 and SW2 are both connected to a upstream MX. They are advertising their bpdu correctly but upon inspecting the bpdu I noticed that SW1 Port 1 is advertising a Port identifier of 0x8201 and SW2 Port 1 is advertising a Port identifier 0x8101. 

To keep everything simple this causes issues because it results in a suboptimal path. 

I was expecting SW2 or SW3 or any other switch in my stack to have a Port identifier HIGHER than SW1. 

How is the port identifier based in a switch stack ?

Thanks ! 

[RaphaelL]

For those ( like me ) who likes a good picture to understand better : 

[ww]

I dont know.. Is the lowest portid  the switch with the lowest mac address?

[RaphaelL]

Actually yes... SW1 has a MAC : 34:56:fe:96:b3:84 and SW2 : 34:56:fe:26:bc:5e   but the switch master of the stack is actually : 0c:8d:db:bb:93:58 ( it is also the root bridge advertised via stp ) 

[RaphaelL]

So , just a little heads up. It seems that the portID is based on the  MAC ( from the lowest MAC to the highest MAC ) and not from the order of the stack : 

Which then messes up my topology ( the flow is not optimal ) : 

I'm aware that transfering the root to the stack on the right 'would' solve the issue.

Setting the portID from the physical order of the switch ( from sw#1 to sw#X ) would also solve the issue and I'm pretty sure that is how the rest of the industry is handling the portID ( to confirm )