STP Confusion: 4 Switch Network

[yodaddy0125]

Good Afternoon,

I am trying to get some assitance on determining theoretically the correct way STP will block the ports on a 4th switch in a simple network topology. I know the tie breaker rules just dont fully understand them for the bottom switch. We are being taught that the bottom Switch bases it decision off of rule 4 (Lowest Port ID) because rule 3 Sender Bridge ID is local to it self. So FA 0/1 will be the DP and FA 0/2 will blocked on bottom switch. Instead of the other way arround, is this correct and is there any way to better explain this.

I assumed that FA 0/2 would be the Root port and FA 0/1 would be blocked becuase the right siwthch has a lower bridge id and that is where the BPDU are originating from.

[GRANT3779]

Has this been labbed up and tested to see which port does become root?

What are the Switch priorities of the switch on Left and Right? Are they the same / default setting?

My understanding from the given topology is that your bottom switch will currently have equal cost to the root bridge from each port.

If this is true then the next process for root port selection is to select the port that has the neighbour with the lowest Bridge ID. Assuming default priorities then Fa0/2 would be Root Port.

If Fa0/2 is not root port then I would double check the switch priorities on two side switches (remember the Bridge ID is made up of 2 parts)

[yodaddy0125]

All switches in the topology have the same priority which would be the default. The diagram is labled correctly. All path costs are identical. 

We are being taught that FA0/1 would be root port just because the port id is less on the local switch that has to make the determiniation of which port to make the root port. The Sender Bridge ID that is used in the STP tiebreaker process is the same as the Local switch bridge ID (Bottom Switch).

I was taught years ago, and did this same expierement with (4) 3750's and got the same results that your are stating and what i tried to explain to professor. That Port FA0/2 of bottom switch will be the root port and port fa 0/1 would be blocked.

[GRANT3779]

I have just labbed this up in Lab Builder and tested with different priorities for the switches 2 and 3 in my diagram. Only testing very basic 1 Vlan setup and amending priority with the following -

spanning-tree vlan 1 priority x

It worked as I thought it would explained below. If costs are equal then port towards lowest sending bridge ID is slected as root in the topology.

I would lab it up and see for yourself.