Port fast spanning tree

[Rishav Garg]

Hi All,

What happen when port fast enable port start receiving BPDU from other end.

Is it loose port fast capability and start working as normal port and after listening learning it will block the port. or is it work as a port fast to allow BPDU so that loop might be occur?

Regards

Rishav

[Vasilii Mikhailovskii]

Hello.

Portfast does not disable port on BPDU arrival.

If you want this behaviour, then you need to configure BPDU guard along with portfast.

[Jon Marshall]

Rishav

If a portfast enabled port receives a BPDU it will transition to a normal port and go through the listening and learning processes of STP.

Jon

[Peter Paluch]

Hi Jon,

If a portfast enabled port receives a BPDU it will transition to a normal port and go through the listening and learning processes of STP.

This is what most textbooks say, and this leads you to believe that when a PortFast-enabled port receives a BPDU, it will fall back to Listening, thereby stopping the traffic flow, and will need 30 seconds to become Forwarding again.

In reality, what happens is that the port will consume the BPDU, disable the PortFast on the port, but whether the port truly changes its role and state depends on the contents of the received BPDU. If it is inferior to that port's BPDUs, nothing will happen at all, as the port will remain Designated Forwarding. If it is superior to that port's BPDU then the port will become either Blocking (i.e. Alternate Discarding) or Root Forwarding, depending on how good the incoming BPDU is.

Best regards,
Peter

[Jon Marshall]

Peter

Thank you for clearing that up.

I obviously need to do some more STP labs.

Jon