portfast vs disabling spanning-tree

[jon_panes24]

hi,

could someone give me an indepth explanation why enabling portfast is not the same as disabling spanning tree with regards to detecting loops?

thanks 

[Madhukrishnan Gopinathan Nair]

Hi,

Disabling spanning tree and port fast are different. Port fast helps to transition from Blocking state to Forwarding state immediately, but it is still running spanning tree. So if it receives bpdu from other side it will loose it portfast status and become a normal spanning tree port. 

But disabling spanning-tree is like inviting trouble. Even if you are sure that there is no switches going to be connected to certain ports, there are chances sometime by human error/lack of knowledge end users may connect some switches or hubs or sometimes introduce physical loops. So if you have spanning tree disabled, you are going to have a tough time facing it :)

Hope this helps and grade the posts which are useful

Thanks,

Madhu.

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

With STP disabled, there's no loop detection.

With Portfast enabled, there's a chance of loop detection.  The reason for "chance", a L2 loop could crush the net before a Portfast port's STP reacts.  This is why STP listen and learns before it opens the port to normal traffic.