PortFast in a trunk port

[fabio.marino]

Hi to all, i was no able to understand the reason behind portfast configured on an interface that is in trunk mode.

Usually, i know that port fast is used on access port in order to speed up the transmission of data by a final host, so whici is the function of port fast in an interface in trunk?

Thanks, Fabio.

[Shashank Singh]

Hi Fabio,

Portfast enables a port to skip LIS, LRN states of STP and transition directly from  BLK to FWD mode. This saves convergence time on this port and the

device connected here (normally a host) doesnot have to go wait for 15+ sec to be up.

Though it is supported on trunks also, it is never advised to use it. If at all used, it will have the same behavior.

Skipping LIS, LRN states will not flush out and relearn mac addresses associated with this interface even if those mac addresses have been learnt on other interfaces (due to a topology change) and will result in spanning tree loop as the port goes into FWD state even if needs to be blocked by STP.

Router(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

Cheers,

Shashank

[Mauricio Naegele da Cruz]

If you are configurin a server in some vlans (trunk) and want to speed up convergence you can use portfast trunk, for example !!!

Best Regards,

Mauricio

[Latchum Naidu · Accepted solution]

Hi Fabio,

You can use PortFast to connect a single end station or a switch port to a switch port. If you enable PortFast on a port that is connected to another Layer 2 device, such as a switch, you might create network loops. But you can enable portfast with router on stick case.where you have switch with trunk configured with router interface.

And Yes, you are right in what you say ie. you shouldn't use portfast on a trunk link between switches. Where i have seen the portfast trunk command used is when you have a server that supports 802.1q trunking and is running multiple vlans on it's NIC.

Please rate if this helped you...

Regards,
Naidu.

[fabio.marino]

Thanks Naidu, you got my doubt, I need a practical example of using trunk and portfast at the same time. Thaks for your time. Bye, Fabio.

[Latchum Naidu]

Hi Fabio,

I am glad to see that your doubt has cleared.

And you are most welcome

Regards,

Naidu.