enabling port fast 1

[skydiver3535]

newbie question here - are there any benefits by enabling portfast on a cisco switch? small network - straight connection from the desktop to a port on cisco switches series 3500. i ask this because on all the switches this feature is disabled - i have several laptops that if the user logs on fast, they dont get the logon script - the nic driver seems to load and negotiate with the dhcp server after the logon process ends so users dont get proper network access. someone suggested to enable portfast so the negotiation process happens fast enough - are there any issues by enabling portfast? any ideas
thank you

 

[cisconooblet]

Not on an interface connected to a single pc. Go for it.

 

[cisconooblet]

Benefit is as soon as the link comes up the switch will start forwarding information, instead of waiting through the learning, listening stages before transitioning to a fwd state. You will be fine on an end user interface to enable it.

 

[cisconooblet]

oh last thing is I recommend putting bdpu guard on the interfaces that you enable port fast on. Keeps someone from plugging in a new switch and negotiating as root. Make sure you have them set to access as well.

 

[skydiver3535]

sounds good - i will enable portfast on these interfaces then -
can bpdu be enabled per single interface?

 

[cisconooblet]

yes. its an interface command.

 

[unclerico]

I see that nooblet has this pretty much handled, but I wanted to add another benefit to enabling portfast on all edge ports and that is the propagation of TCN BPDU's or lack thereof. What I mean is that if you have an end station connected to a port without portfast enabled anytime that port is moved into forwarding state or to blocking state a TCN BPDU is generated and forwarded to the root bridge. When the TCN BPDU is propagated to the other switches they will shorten their MAC address aging timer to foward delay (default 15 seconds) from the default of 5 minutes. This can create instability in your network if you have a lot of end stations coming up or going down. Ports configured in portfast mode do not generate TCN BPDU's.

you can enable both globally:

spanning-tree portfast default
spanning-gree portfast bpduguard default

or at the interface level:

switchport mode access
spanning-tree portfast
spanning-tree bpduguard enable

Be sure to award nooblet a star for the help

I hate all Uppercase... I don't want my groups to seem angry at me all the time! =)
- ColdFlame (vbscript forum)