Why Portfast?

Guy Couturier

Studying for 200-301.. just got through your STP Portfast/BPDUGuard video.

I'm curious as to why one would want to enable the PORTFAST and BPDUGUARD settings.

I understand the protections they provide. In the video you said they could be used instead of STP to decrease the time and resources used within the switch when connecting a port to something that will never need STP such as a workstation.

But when one considers how quickly RPVST moves, wouldn't it be safer (and fewer trouble tickets to recover) to use RPVST rather than take the chance that someone plugs the wrong type device into the wrong port? And as far as resources.. it would only happen when connecting a device to the port... mostly that happens one at a time. The only time that would REALLY be taxing is when the switch is powering up for some reason. And if the switch is already doing that.. there were bigger problems to worry about than a temporary crowding of resources. Right?

Ronnie Wong

@Guy-Couturier,

So if we use portfast, the STP pretty much doesn't have to run through it...but if we want more then enabling BDPUGuard says if we even hear it...do your thing. Granular control too, BPDUGuard has the ability to be enabled/disabled per interface or globally along with portfast.

The question then is what happens in RPVST when a switch is plugged into an "edge" port? Once you answer that question... (answer: blocked) which means it can recover again by running through Spanning-tree.

The other question is what happens when a switch is plugged into a BPDUGuard and Portfast enabled port? (spanning-tree portfast bpduguard) the answer is the interface is put into err-disabled state?

So It is not a matter of safety per se, but a matter of how you want the issue to be handled? Do you want to know if someone is trying to plug something into a open port that shouldn't be there and have to investigate or do you want the switch to just handle it.

I hope that helps. We normally don't get that deep into this at CCNA because there's so much that candidate just already do.