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STP port roles help

cisconetguy
Level 1
Level 1

Hi, I am having lot of confusion understanding STP Port roles.

I know, How STP determines the root bridge, but having understanding in Root port determination, and designated/Non-designated ports.

I have uploaded a topology, please see.

1. In topology, Sw1 is the root bridge.Sw1 sends root path cost 0 to Sw2, and Sw3. When Sw2, and Sw3 receives the BPDU, Do they add cost of 19 on incoming interfaces or outgoing interfaces? I am really confused in this.

2. The segment between Sw3, and Sw4, Sw3 fa0/2 is designated, and Sw4 fa0/3 is root port? why is this? Why Sw4 fa0/3 is root port? Is it because of lower root path cost to root bridge? If that's the case, Why it didn't select Sw4 fa0/4 as root port?

3. When STP considers lower BID, What exactly it compares inside that BID?  As I know, When choosing Root bridge, It considers priority+ mac.

    What about when choosing designated and non-designated port? Does it consider also priority+mac address too?

Could you please answer my questions in good explanation?