Wud overall stp convergence take place if access port flaps having pc connected to it.If port fast is enabled then it should transisition immediately to forwarding and overall stp convergence should not take place.Any suggestion???
your understanding is correct: if you use STP portfast no topology change notification BPDU is sent from switch upwards in direction of root bridge and STP doesn't need to recalculate.
The usage of STP portfast = to define ports as edge ports is also important when using Rapid PVST or MST 802.1s: it helps in achieving fast convergence by reducing dramatically the number of links where synchronization process occurs (the process of port state negotiation using BPDUs handshaking)
u mean if we dont put port fast on access port stp convergence will be triggered overall and TCN will be sent towards root bridge.I don't think so that will happen, rather port will take 50 sec to be in forwarding state if there is no portfast on access port connected to end station.
"u mean if we dont put port fast on access port stp convergence will be triggered overall and TCN will be sent towards root bridge. I don't think so that will happen, rather port will take 50 sec to be in forwarding state if there is no portfast on access port connected to end station."
Both will happen. The port will indeed take approx 50 seconds with standard 802.1d timers.
But more importantly without portfast a TCN will be generated each time the port transitions from up to down or down to up which you certainly do not want within your switched network. So portfast really is an important part of your configuration for edge ports.
A TCN is the not the cause of a change in the STP topology, rather it is a result of a change in the STP topology.
So a TCN does not trigger an STP recalculation but it does trigger the flushing of the CAM tables on the switches. There is no need for this flushing every time an end device such as a host transitions from up to down and down to up.
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