STP algorithm question

[Kapfel]

Portfast puts an access port in which it is configured on directly into forwarding state. In addition loop detection is still in operation and TCN BPDUs aren't sent if the interface goes up or down. So my question is does the spanning-tree algorithm actually run everytime the link changes state?

Thanks!

 

[vipergg]

Yes if you don't have portfast turned on for the given port. Generally all client ports should have portfast turned on and and inter hardware connections (switch to switch, switch to router ) should not have portfast turned on .

 

[Kapfel]

Thanks for the reply Vipergg. I actually didn't state my question correctly. The question is does the spanning-tree algorithm actually run everytime the link changes state when portfast is enabled on the link?

Thanks

 

[valarian]

Hi Kapfel, link state changes of ports which are configured with portfast do indeed trigger spanning-tree. Portfast only causes a spanning tree port to enter the forwarding state immediately, bypassing the listening and learning states. It is only supported on nontrunking ports though. Ports configured with PortFast, that recevie BPDUs are immedialtely put into blocking state by spanning-tree. In a valid configuration ports configured with PortFast do NOT receive BPDUs. When activating the PortFast BDDU Guard feature on the Cat, a PortFast-configured port that receives BPDUs is shut down instead of being put into blocking by spanning-tree.

Mike