While reading a bunch of details about STP timers on this Cisco website https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/19120-122.html#f1
I could not understand how they ended up with the formula for end to end propagation delay.
Here is the formula from the website
End-to-end BPDU propagation delay—This value is the amount of time that is necessary for a BPDU to travel from one end of the network to the other end. Assume a diameter of seven hops, three BPDUs that can be lost, and a hello time of 2 sec. In this case, the formula is:
End-to-end_BPDU_propa_delay = ((lost_msg + 1) x hello) + ((BPDU_Delay x (dia – 1)) = ((3 + 1) x hello) + ((1 x (dia – 1)) = 4 x hello + dia – 1 = 4 x 2 + 6 = 14 sec
I dont get how the lost BPDUs influence the formula. They assume we could lost BPDUs but how those BPDUs are recovered actually? Since there is only Ethernet frame, there is no recovery option.
Imagine the following network diagram
SW1 starts sending Hello BPDU (because its interface has been activated let's say. The interface will be in LISTENING status right after sending its BPDUs.
Here is my question: If the BPDU got lost between SW2 and SW3; it will never reach SW4. But SW1 will still changes its LISTENING states to LEARNING after the normal forward delay independantly from the BPDU loss! Thus, the SW1 could learn "corrupted" mac addresses as SW4 is not aware of the topology change and there is no recovery mean. If there is no recovery mean; why the "official" formula includes several packet loss (as if one loss could be recovered by the next BPDU generation)?
What did I got wrong here?
Thank you for your time and expertise,