Portfast Trunk

N3tWrkNut

Does anyone know why you would use this? I am new to a company and in my studies I have read not to enable portfast on a trunk link. I get here and I see that portfast trunk is enabled and I can't seem to find any good reason to have it enabled and the only reason I get for it being enabled is because the vendor enabled it.

Help or guidance please!

down77

CLI tells you why:

6509(config-if)#spanning-tree portfast trunk
%Warning: portfast should only be enabled on ports connected to a single host. Connecting hubs, concentrators, switches, bridges, etc… to this interface when portfast is enabled, can cause temporary bridging loops.
Use with CAUTION

Regards

N3tWrkNut

I have seen that statement when I put it on an access port. That is why I am asking why would you use it on a trunk port especially to another switch. That is how it is setup here and it makes me nervous.

N3tWrkNut

Never mind....I see what you are saying now. Reading is very important....I still see a port that is trunked to a switch with portfast trunk....I will have to remove that one since the others do not have that. The others go to the esx hosts hense the single host

Thanks for your reply!!!!

down77

Typically your trunk port is connected to another switch which means you may run the risk of creating a spanning-tree loop... which in turn could temporarily take down your segment or network.

Maybe they have a need to bypass STP directly into the forwarding state for convergence? Or it could be for IP Phones?

Found an older thread with the same question:

http://www.techexams.net/forums/ccnp/38674-spanning-tree-porfast-trunk.html

elphrank0

Portfast reduces the amount of time for convergence if I remember correctly. It would stop the 50 seconds it takes for convergence and do it in like 5 or 10..

elphrank0

"Using it means the port won't spend the usual 50- seconds to come up into forwarding mode while STP is converging."

Lammle, Todd, 2007, CCNA Study Guide, 6th, PG. 512

GT-Rob

We use portfast on trunks all the time.


On our blade switches, the interfaces make a trunk with the server. Since its impossible to have a switch there, and the server needs portfast for DHCP/PXE to work correctly, we need it on.


If you were to connect a switch to a trunk and were 100% sure there were no loops, there is no problem with port fast (or bpdufilter/guard if you wanted).