spanning-tree portfast trunk

[mahesh18]

Hi all,

i read that portfast should only be enabled on access ports  not on trunk ports.

when this command is used

spanning-tree portfast trunk?

under what cases we will use portfast command on trunk port ?

thanks

mahesh

[Ganesh Hariharan · Accepted solution]

Hi all,
i read that portfast should only be enabled on access ports  not on trunk ports.
when this command is used
spanning-tree portfast trunk?
under what cases we will use portfast command on trunk port ?
thanks
mahesh

Hi Mahesh,

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.

Hope to Help !!

Remember to rate the helpful post

Ganesh.H

[mahesh18]

Hi Ganesh,

Many thanks for good reply

[devendra.gawade]

Hello Ganesh,

                        when i gone for interview ; they asked me suppose you have one L2 switch. you need to configure trunk port. when you configured trunk port ; before saving the running config you type portfast command ; what will the output ?...

waiitng for your reply.

Thanks & Regards,

Devendra

[Leo Laohoo]

Devendra,

That's a trick question.  I will never, under any circumstances, configure my uplink trunk port with STP portfast. 

[devendra.gawade]

Hello,

             i will be waiting for your answer.

Thanks & Regards,

Devendra

[adgaurav]

This can introduce an Immediate loop in the network. For example if the port is not configured and is down. It comes up and at the same time you configure it as trunk with portfast on, this will not generate a TCN and the port will directly go into forwarding state for all the vlans allowed creating a loop if it is connected to another Layer 2 switch. However if you configure portfast on a already up link(converged topology), a loop will be created until the next topology change in the network.

[Jon Marshall · Accepted solution]

Mahesh

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.

Jon

[mahesh18]

Hi jon.

thanks for great reply again.

just to be courious which servers are these which support 802.1q trunking.

thanks

mahesh

[Jon Marshall]

mahesh18 wrote:
Hi jon.
thanks for great reply again.
just to be courious which servers are these which support 802.1q trunking.
thanks
mahesh

Mahesh

Nowadays it is VMWare servers where you see them participating in multiple vlans but before VMWare it could be any server that you wanted to be in more than one vlan and as long as the NIC supported it you could set it up that way.

Jon

[erisanchez]

You should almost never apply a spanning-tree portfast trunk command to another cisco switch.  If you go back to the basics... spanning-tree portfast allows the ports to skip the blocking to fowarding states and go immediately to forwarding.  The spanning-tree portfast trunk does the same but for trunk connections.  It is recommended to do this on vmware/ESX servers that require a multi-vlan use port, BigIPs, Netscreens, any non PAGP using device.

[mahesh18]

Hi,

Thanks for more explanation

Mahesh

[jumora]

I know that this is an old post but then again I was looking for something else and crossed this conversation about portfast. You can actually enable this also against ASA firewalls that have sub interface configuration against a trunk port on a switch, specially when you have ASA failover.

[mahesh18]

Hi Jumora,

Thank for updating me on this.

No post is old.Everyday we learn something new.

Best regards

MAheshg

[Dilshad Ahmed]

Hi Jumora, I think i have something similar to what you have described.

FW1---SW1---SW2---FW2 (FW 1 and 2 are also connected via failover link and have failover enabled)

When the primary switch fails we see some drops, but when it comes back we see a crazy number of drops. Could it be linked with ASA failover convergence? The host that i am trying to ping is behind FW.

I have rpvst with sw1 hardcoded to be root and don't have portfast trunk on link to FW.

If you could point me to some discussions/docs describing this scenario it would also be great.