Can someone explain how loopguard works properly using my topology as an example?
My topology is in the link below. Consider that everywhere is fibre (hence the send and receive)
http://gyazo.com/a1d7a1e43d1ed7aa5765773836633df9
Imagine SwA is the designated port for this segment and SwB is blocking.
So say the top link between the two multiplexors dies. SwB will expire the BPDU stored in his fibre port after a duration of the max_age timer, then transition his side into designated forwarding. So both sides then become designated forwarding. There still can't be a loop in the topology because the flow of traffic from SwA towards SwB is blackholed on the multiplexor. So SwB can't possibly receive traffic from SwA. Therefore, what's the purpose of loopguard? That's suppose to protect the STP against this type of problem, but there's no loop = no problem. So I can't understand its purpose.