STP: Confusing with designated and alt port

[josefernando]

Hello community, I'm a little bit confusing with this topology

I don't understand why G0/2 of switch B is in BLK state, the theory says when exists an identical root path cost, stp choose the dp by this 4 conditions:

1. Lowest root bridge ID
2. Lowest root path cost to root bridge
3. Lowest sender bridge ID
4. Lowest sender port ID

But from my point of view, in my topology, doesn't exist an identical root path cost to SwitchC, or maybe I'm totally wrong, so pleaseI'll very appreciated if someone of you could explain me this misunderstanding.

[Jon Marshall]

For the DP election it is the cost advertised onto the network segment that counts. 

So looking at your diagram switch A receives a BPDU from switch C and adds the cost of the incoming port which is 19 and then advertises that BPDU with that cost to switch B. 

Switch D receives a BPDU from switch B and adds cost of incoming port which is 19 and advertises to switch B. Switch B receives that BPDU and adds the cost of the incoming port which is 4 to 19 which equals 23 and advertises that to switch A. 

So switch A has advertised a cost of 19 onto the top network segment and switch B has advertised 23. 

Least cost wins so switch A's ports becomes DP and switch B's port has to block. 

Jon

[josefernando]

Thanks for your valuable answer, I was searching more answers and fortunately I found the same case in this web 9tut.com and says: "Now our last task is to identify the port roles of the ports between SwitchA & SwitchB. It is rather easy as the MAC address of SwitchA is lower than that of SwitchB so Fa0/2 of SwitchA will be designated port while Gi0/2 of SwitchB will be alternative port ".

So it seems stp is choosing the DP by the BID, is this right or wrong?

Link: http://www.9tut.com/ccna-stp-questions/comment-page-6

Question #6