I am trying to get a thorough understanding of what does happen when a switch is connected to a portfast port on another switch that is part of a stable STP network in case of:
1- the port has no bpduguard.
2- the port has the bpduguard enabled.
I know that in case 1, loops can be formed. but why? And does the port really revert to normal STP operations (LSN/LRN/FWD) as soon as it receives the first bpdu? if so, it technically prevents data forwarding as soon as it receives the 1st bpdu until FWD is reached. In this case, how is this different from bpduguard that will do the same thing (preventing forwarding) after the 1st bpdu?
And in both cases, there could be a duration of time before the 1st bpdu hits the port in question.
So, isn't the loop danger present in all cases?
I know that bpduguard has definitely much better protection by disabling the port completely. However, how is that better than reverting the port back to STP?
I looked on the Internet and even on the SE, however, nothing was clear enough or explanatory of the process in both cases (step by step) as if we are using a Wireshark. Unfortunately, I do not have a new laptop that can handle GNS3 to emulate this myself. and in Packet Tracer, no loops form at all. I guess because no real traffic.
Anyhow, I hope I will find a satisfying answer here.