Find the Alternate (Blocking) Port of a Ring Topology

Examine the topology and try to predict which port will be blocked in STP?
Most of the networking guys will say it is the e0/2 of SW-3..
Let's find out..
As you can see, CORE will be the root obviously according to the configured priority values..
Next step of STP will be to find the Root ports per non root switches..

Obviously e0/2 of SW-1 and e0/1 of SW-3 will be Root ports because they receive the best BPDU from root because all links have equal costs..

Now let's see, what will be the Root port of SW-2..

SW-2 will receive 2 BPDUs from SW-1 and SW-3.
As you can see, I have named them as BPDU 1 and BPDU 2. Let's analyze the contents of them to find out which will be the superior BPDU..

BPDU 1 -
Root Bridge:- 1
Root Path Cost:- 100
Sender Bridge:- 4097
Sender Port:- 128.1

BPDU 2 -
Root Bridge:- 1
Root Path Cost:- 100
Sender Bridge:- 8193
Sender Port:- 128.2

Because the BPDU 1 has the lowest Sender Bridge ID, it will be the superior BPDU hence e0/2 of SW-2 will be the Root port..
Now it is the time for STP to find out the Designated Ports per network segment..
Obviously all the ports of CORE will be Designated ports as it is the root..
Also the e0/1 of SW-1 will be the Designated port of SW-1 to SW-2 Ethernet segment as the other end is a Root port (which sends the superior BPDU to that segment).

So the real problem is in the SW-2 to SW-3 network segment.

Let's see which one sends the superior BPDU to the segment by analyzing the contents of those BPDUs..

BPDU 1 -
Root Bridge:- 1
Root Path Cost:- 200
Sender Bridge:- 8193
Sender Port:- 128.1

BPDU 2 -
Root Bridge:- 1
Root Path Cost:- 100
Sender Bridge:- 32769
Sender Port:- 128.2

Well, because of the lower root path cost of BPDU 2, it becomes the superior BPDU hence the e0/2 of SW-3 will be the Designated port for the segment while the e0/1 of SW-2 will be the Alternate (blocking) port..

If you want to know how the STP really works please go here.