Can someone explain me in detail how the STP decides on the location of the blocking port in a square/circle of switches?
I have attached a real life example. Why is it blocking on the indicated port on that segment and not on the segment between the top left switches for example?
Pascal, you are not stupid...you are asking excellent questions.
Each switch has to make a decision regarding which port to block. It will block the port that is receiving inferior BPDUs.
A BPDU is superior than another if it has:
A lower Root Bridge ID.
A lower path cost to the Root.
A lower Sending Bridge ID.
A lower Sending Port ID.
Using that algorithm, each switch will want to block the port that receives the inferior BPDU. However, its the switch that converges first that will block one of its ports accordingly and break the loop.