Why do STP PortFast ports transmit BPDUs?

[Mitrixsen]

Hello, everyone.

I am studying about STP and my current topic is Portfast.

There's one question I have regarding it. Why do Portfast-enabled ports transmit BPDUs out of them? Isn't the idea here that such a port will only connect to end hosts or to device that do not generate them?

Is it correct to say that the reason why they do this is in a situation where we plugged another switch into it? Because from my understanding and labbing, if we plug a switch into a portfast port and the port receives a BPDU, it will become a regular STP port (if BPDUGuard is not enabled of course).

David

[Martin L · Accepted solution]

Yes, but just in case someone accidently (or not) attaches another switch (managed or not) or hub, port will go via STP process and ends up in blocking state. Portfast-enabled port does not generates topology change notifications which is the main reason for Portfast, I think.  With BPDU Guard and portfast, port will be disabled (err-disabled) and could be recover (came up) automatically if u wish to do so.  

Regards, ML
**Please Rate All Helpful Responses **

[Joseph W. Doherty]

"Portfast-enabled port does not generates topology change notifications which is the main reason for Portfast, I think."

Yes, that and the pregnant delay for an edge host to join the network after the port goes up.  If fact, not uncommon for some hosts to give up waiting on DHCP because they see link up but no DHCP offers.

[Joseph W. Doherty · Accepted solution]

You got it all correct.

Portfast ports try to still preclude creation of accidental loops, but I emphasis the word "try".

[MHM Cisco World]

Stp port send receive bpdu

STP control send only by using bpdu filter which prevents port send bpdu but cisco not recommended that because it simply disable stp in port

STP detect bpdu receive, this divide into

Short timer stp using portfast 

Protect using bdpu guard 

MHM