What is exact effect of portfast port connected with a switch in loop ?

[wfqk]

Hi What is exact effect of portfast port connected with a switch in loop ? If a new switch is plugged into portfast port in another switch, the effect  could be the new switch will bother STP of core. Is there any effect else ? How can this cause a loop in the network ? I do not understand how to cause the loop without physical loop. Anyone can help to give me some suggestion ? Thank you

[Leo Laohoo · Accepted solution]

Go HERE.

[Joseph W. Doherty]

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A portfast port still runs STP, so if there's a loop, the port should block.  However, while a non-portfast port insures a loop free topology before the port is used to forward traffic, a portfast port does not.  This means a loop can be active before the portfast port blocks and it also means, a very severe loop might overload the switch before STP can block the port.

Because of the latter, portfast should be assigned with great care and something like BPDUGuard is also configured on such ports too.