in RSTP when bridge assurance is NOT used only designated ports speak and non designated ports listen to RSTP BPDUs originated by the root bridge and delivered on other switches.
local switch --RP DP-> upstream switch --RP--> <--DP---root bridge
<----- direction of BPDUs in normal conditions
IF there is a fault between the switch where the local root port is connected (this is the upstream switch ) that makes the upstream switch unable to receive BPDUs on any port the upstream switch can only skip to send the Hello configuration BPDU to the local switch.
The local switch has not a direct failure the link is up but it is not receiving BPDUs from the upstream switch on its root port.
In this case the local switch after 6 seconds can try to react and to elect a new root port.
In old STP the switch had to wait 20 seconds (max-age) before reacting to an indirect failure and then going via Listening and Learning state it could require 50 seconds for convergence.
Cisco introduced for classic STP an optimization called backbone fast that allowed to skip the max-age timer.
This optimization is now integral part of standard RSTP.
if you unplug the fibers or RJ 45 cable the link will go down and there is no need to wait for 6 seconds to react.
This why you are not losing a packet in ping tests.
Hope to help
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