STP Port Priority

Hello,

I'm wondering how STP works out the Port ID part of the port priority...

Say we have four switches

<SW1>--------------- <SW3>
    | |
    | |
    | |
    | |
<SW 2>---------------<SW4>

Say SW1 + 2 are running MST Region name 12

Say SW3 + 4 are running MST Region name 34

The Intra Region Links on both switches are port fa0/1
The Inter Region Links on both switches are port fa0/2

The Port Costs between the regions are the same.

SW1 is the CIST ROOT.

Either SW3 or SW4 will elect the Boundary Root Port towards the CIST Root
based on the lowest cost and become the CIST Regional Root. Now from what I
understand that Port Cost for CIST is only cumulative between boundary ports
and will not be adjusted within a region. So then if SW3 and SW4 have the
same link costs there is going to be a tiebreaker, which would have to be
broken
based on Bridge ID.

The Bridge ID in MST being the CIST Regional Root Bridge ID (Which I am
assuming going off the MSTP BPDU which has the CIST Regional Root ID in
place of where the Bridge ID used to be)...

Further down, I then see that we have the "CIST Bridge ID" it looks as if
these a additional fields for MST. So this would likely act as the
tiebreaker for who becomes the CIST Regional Root in Region 34.

Which brings me to my next question.

If SW3 / SW4 was then running PVST+ it would not understand the "CIST Bridge
ID" and Region 12 would look like a single Virtual Bridge. This is fine,
except what if then my Port Priority and Port ID's are the same?

What is the tiebreaker?

Thanks,

Garth

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Received on Tue Sep 07 2010 - 16:41:13 ART