question about spanning tree portfast and bpdu

[syedhasnain11]

My understanding is that by default, PortFast is disabled on all switch ports and you can configure PortFast in two ways: per port and globally.

If you configure PortFast globally, all ports that are operational as access ports automatically become PortFast enabled and if they receive a BPDU, they will go into blocking mode.

If you configure PortFast on a port, the port will become PortFast enabled.

My question is, what happens when a BPDU is received on a port which has been enabled with PortFast under interface and not globally. Does the port go into blocking state, same like when PortFast is enabled globally?

Thanks
Syed

[ngkin2010]

Hi,

If you configured the PortFast in interface specific mode, then you simply disabled the STP on that interface. It would not go through the spanning tree states (BLK , LRN, FWD).

[Sergey Lisitsin]

ngkin2010,

That is actually incorrect. Enabling portfast on an interface doesn't disable spanning tree on it. If a port with portfast enabled receives a BPDU it will process it as normal. The difference between portfast-enabled port and non-portfast-enabled is that portfast-enabled port skips listening and learning states and goes straight to forwarding.

[ngkin2010]

Hi,

Thank you for pointing it out. And I think I have to refresh my knowledge..haha. Thanks again.

[omz]

Hi, 

Please check below link - good post by Daniel Dib. Hopefully, it will answer your question. 

http://lostintransit.se/2013/08/04/busting-myths-spanning-tree-portfast-on-the-interface/

[paul driver]

Hello

Portfast applied either global or interface - will go from blocking and then bypass listen/learn (pvst+) states  or from discarding and Learning states (Rpvst+/mst) and go into forwarding.

[Joseph W. Doherty]

In addition to what's been correctly noted (i.e. a PortFast enabled port will go into non-PortFast mode if it "sees" an BPDU), I understand that you do risk a L2 loop bringing the device(s)/network down. I.e. a L2 loop might consume the switch before it can block the loop.