RSTP ports

I am having hard time getting the understanding on RSTP algorithm for the following switch config (See attachment). Please let me know which ports that have role in RSTP  for switch & int's in figure attached:

Switch-config.gif
totaramAsked:
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donmanrobbCommented:
P4S-SC will be the root switch because it has the lowest mac address.
P4S-SA F0/1 will be the root port because it is the lowest interface number, F0/2 will be the designated port.

PFS-SD, Gig0/2 will be the root port due to lowest cost.
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Don JohnstonInstructorCommented:
There's a problem with this question.

The switch with the lowest bridge ID is the root. The bridge ID is made up of a two byte bridge priority (default of 0x80.00) followed by the base MAC address of the switch. In the question, they state "No other configuration changes have been made."

This means that P4S-SC is the root. All ports on the root are Designated Ports (DP).
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Don JohnstonInstructorCommented:
Sorry... hit the submit button by accident.

So the BPDU's will arrive at SA on port Fa0/1 with a cost of 19. This means that Fa0/1 on SA will be the Root Port (RP). This means answer B is correct. Fa0/2 will be a DP which means A is correct also.  

Now on SD, we could assume the gig0/2 port negoiated to 100mbps which would make the cost of that port 19 also. So G0/2 will be an RP so D is correct.

That's three.

But now we continue to follow the BPDUs.

Since G0/2 on SD is the RP, G0/1 will be the DP.

On to SB:

BPDU's arrive on both G0/2 and G0/1. If we assume the G0/2 negoiated to 100mbps, the BPDU will have a cost of 38 (19+19). The BPDU arriving on G0/1 will have a cost of 23 (19+4). Since the RP is selected based on the lowest cost, G0/1 will be the RP. Which means C is correct.

So A, B, C and D are all correct.

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totaramAuthor Commented:
A couple of thoughts:
1. Fast ethernet vs gigabit ethernet has to figure somewhere in the computations.
2. Donjonston, A is not somehow not correct.
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Don JohnstonInstructorCommented:
1. Correct. fastethernet ports have a default cost of 19. Gig ports are 4. If you connect a gig port to a fa port with default configurations, the gig port will drop to 100 meg and have a cost of 19.

2. Sorry. A is correct.

More than likely, the rocket scientist at Pass 4 Sure goofed up. This wouldn't be the first time.
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donmanrobbCommented:
You dare suggest that the elite professionals at Pass4Sure made a mistake? :)
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Don JohnstonInstructorCommented:
I'm daring kinda guy.  :-)
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totaramAuthor Commented:
Hi Don;
You say A,B,C & D are correct...

That leads to switching loop, since all the ports are forwarding. There has to be some port that is blocking.
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Don JohnstonInstructorCommented:
There are only 4 answers but there are 8 ports on the diagram.

Switch B G0/2 will be a non-designated (blocking) port.

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donmanrobbCommented:
Read my answer again, I didn't tell you to choose all answers :)
P4S-B Gig0/1 will be the blocked port.
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donmanrobbCommented:
typo, P4S-B Gig0/1 is blocked
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Don JohnstonInstructorCommented:
I have to disagree with you donmanrobb.

Switch B will have two possible paths.

The G0/1 will have a cost of 23 while G0/2 will have a cost of 38. Since cost is the first factor considered in determining the port state, G0/2 will block since it has the highest cost.
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Don JohnstonInstructorCommented:
I can't type fast enough. :-)
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donmanrobbCommented:
donjohnston, right you are.
I tried to correct my typo and put G0/2 but since its early morning and I have not had coffee yet I appear to have wrote g0/1 again.
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totaramAuthor Commented:
Thanks...
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Don JohnstonInstructorCommented:
Totaram:

Have you run across very many IPv6 questions on your Pass4Sure package?
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totaramAuthor Commented:
Not very many, a few... why did you ask that, let me know if some of those.. I am looking for as many questions as possible???
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Don JohnstonInstructorCommented:
I was just wondering if they had gotten around to adding IPv6 to the pool of questions.
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