Root port election - last tiebreaker

I have a question.

For root port election in classical STP the tiebreakers are:

- based on lowest cost

- then based on lowest upstream BID

- then based on lowest upstream PID

What if all those 3 values are the same, what is the next tiebreaker? I searched in Cisco.com, and watched Brian ATCv5 but could not find an answer to that.

I assumed that the next tiebreaker will be the local port ID (because that cannot be the same) but id does not work, i mean changin the priority locally in that scenario will not change the root port.

Thanks.