Hello Quirik
Switch ports in the blocking or alternate state still receive BPDUs from the connected switch. So for the following statement here is the explanation:
So before port C on SW2 went down, when STP was converged and stable, port B had received some BPDUs from SW2. These are the “previously received BPDUs.” When the topology changed, SW2 sent new BPDUs with new information, stating that it is the root bridge. SW3 does not process these yet, until the blocking timer expires.
What makes the superior? I assume you are referring to this statement:
The statement says that once the blocking timer expires, the newly received BPDUs are considered. However, they are not accepted because those received from SW1 are superior to those received from SW2. So, SW1 becomes the root once again, as it should be.
Does that make sense?
I hope this has been helpful!
Laz