spanning-tree portfast vs spanning-tree portfast trunk

[speedfreax]

Hello,

Can anybody help with the following:

What is the difference between 'spanning-tree portfast' and 'spanning-tree portfast trunk' configured on an access-port. What does the command 'spanning-tree portfast trunk' do on an access-port?

Thanks,

Rene

[Peter Paluch]

Hello,

On an access port, both spanning-tree portfast and spanning-tree portfast trunk cause the port to act as a PortFast-enabled (i.e. edge) port. Such a port will immediately become Forwarding after coming up.

These two commands differ if the port is operating as a trunk port (either by static configuration or via DTP negotiation). The spanning-tree portfast has no effect on trunk ports. If you want to make a trunk port become Forwarding imediately after coming up, you need the spanning-tree porfast trunk command. Note that the RSTP has its own mechanism of putting a trunk port into the Forwarding state rapidly, and thus, the spanning-tree porfast trunk command is suitable only on trunk ports leading to Layer3 devices, e.g. routers or servers.

Best regards,

Peter

[Latchum Naidu]

Hi Rene,

First of all PortFast is Cisco proprietary.
PortFast can negatively effect DHCP services.

PortFast is used to more quickly prevent and eliminate bridging loops.
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.

And Spanning-tree portfast trunk should never be used on switch interconnects because with switch interconnects you want STP to make sure there is no loop before forwarding.
You can configure This command is really designed for servers which are running 802.1q on their NICs and which can transition to forwarding immediately.

Hope this clear you.

Please rate the helpfull posts.
Regards,
Naidu.

[Peter Paluch]

Hello Naidu,

Thank you for joining the discussion. Please allow me to have a couple of comments:

First of all PortFast is Cisco proprietary.

For STP, yes. For RSTP, no - it is the edge port that is, for backward compatibility reasons, configured as spanning-tree portfast. The RSTP does not have any mechanism to decide itself whether a port is an edge port, therefore, it is up to the administrator of the switch to explicitly declare edge ports as such.

PortFast is used to more quickly prevent and eliminate bridging loops.

I am afraid I do not agree with this statement. It is the STP, not PortFast, that prevents and eliminates bridging loops. The PortFast, when applied improperly, may actually cause bridging loops, therefore it should be used only towards non-bridging devices (i.e. L2-end devices). After all, on Catalyst switches, whenever you configure the PortFast, you get a warning that a bridging loop may occur if the port is connected to another switch.

Perhaps a typo?

Best regards,

Peter

[inɒmoЯ]

Great comment. 
I've always made it a habit to enable and configure bpdugard whenever I can when turning on portfast. it saved me many times when installers call you and say "Hey, the port was up but it went down on its own...!" yeah right. In another scenario, they like to use hard loops a lot in the field

[m-abooali]

HI,

f we wanted spanning-tree portfast under access ports, do we need spanning-tree default in Global configuration too?

Please advsie,

Best Regards,

Masood

[Alex Pfeil]

If you place spanning-tree portfast on the access port, you do not need the global configuration command. The global configuration command is great for when you want every access port to have spanning-tree portfast enabled.

Please mark helpful posts.

[m-abooali]

Thanks so much!

so, would it be a good practice to have spanning-tree portfast default command in global configuration and have only some access ports (as needed) to have spanning-tree portfast under the port configurations?

all Cisco 350G stack of 2 and 3 switches in each node and all connected via trunk (fiber) to a stack of 375012-s-s (all sfps) as distribution. port channel with two links on all and a PO to teh core with two links.

wouldn't port fast create DHCP problems for Merakie and Aeronet APs? - I am not sure but some people talk about it!?

Please advsie,

Best Regards,

Masood

[Alex Pfeil]

We have switches where we have the default command. We also have switches that cannot have portfast on by default because a certain endpoint will not work correctly with it. I prefer to configure each port. That way if I need to remove it, I can easily do so without still having it applied. It is still a useful command if you have a switch where all of the ports will be configured the same besides the uplink ports.
Please mark helpful posts.

[m-abooali]

Thank you Alex. very useful.