STP Root port selection - missing step?

Hello
I'm trying to figure this out something regarding root port selection. The
root port selection per most documents are:

1. Lowest cumulative root path cost (sum of received cost + incoming port's
cost). This step selects the best path.
2. Lowest upstream Bridge ID (designated bridge). This selects a unique
designated switch.
3. Lowest upstream port ID (designated bridge's port id). This selects a
unique switch port, which should become the root port.

My questions is what happens if you reach step 3 and everything is still
equal?

How is this possible? If you have a simple topology like SW1----SW2===SW3,
where

- SW1 is the root with only one connection to SW2.
- SW2 has spanning tree disabled.
- SW3 has two connection to SW2 (Fa0/20 and Fa0/21)

When I did this in a lab, SW3 would see SW1 as the designated and root
bridge. SW3 would always pick the lowest port, in my example fa0/20 instead
of fa0/21. So is there a step 4 to the root port selection process, where
it will pick the lowest port id of the local switch?

Thanks

Tom

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Received on Wed Apr 04 2012 - 23:24:00 ART