Solved: Spanning-tree portfast Question

Patrick McHenry · ‎06-13-2011

Hi,

other than seeing spanning-tree portfast trunk configured on trunk ports and spanning-tree portfast on access ports, what is the difference between the two commands. Are there any built in safety features with Spanning-tree portfast trunk?

Thanks, Pat.

Peter Paluch · ‎06-13-2011

Hello,

By default, if a port operates in a trunk mode (either via static configuration using the switchport mode trunk or by dynamic negotiation with the peer using the DTP protocol), the command spanning-tree portfast is ignored and ineffective. This behavior can be overriden using the spanning-tree portfast trunk form of command.

In general, neither the spanning-tree portfast nor the spanning-tree portfast trunk commands shall be used on ports towards other switches. If a rapid convergence on these inter-switch connections is desired then RSTP or MSTP should be used that has provisions for rapid convergence without causing even transient loops. However, the command spanning-tree portfast trunk is quite usable on trunk ports towards routers (if performing interVLAN routing), servers and other non-bridging devices that would be negatively affected if it took 30 seconds for a port to become Forwarding.

There are no special safety features built into any of these commands, except the fact that if a PortFast-enabled port receives a BPDU, it loses its PortFast status and becomes a normal non-edge switchport. However, by that time, such a strong broadcast storm may ensue that the CPU of the switches may get totally overloaded and the network may be severely impacted.

A judicious use of this command is very, very much recommended.

Best regards,

Peter

View solution in original post

Peter Paluch · ‎06-13-2011

Hello,

By default, if a port operates in a trunk mode (either via static configuration using the switchport mode trunk or by dynamic negotiation with the peer using the DTP protocol), the command spanning-tree portfast is ignored and ineffective. This behavior can be overriden using the spanning-tree portfast trunk form of command.

In general, neither the spanning-tree portfast nor the spanning-tree portfast trunk commands shall be used on ports towards other switches. If a rapid convergence on these inter-switch connections is desired then RSTP or MSTP should be used that has provisions for rapid convergence without causing even transient loops. However, the command spanning-tree portfast trunk is quite usable on trunk ports towards routers (if performing interVLAN routing), servers and other non-bridging devices that would be negatively affected if it took 30 seconds for a port to become Forwarding.

There are no special safety features built into any of these commands, except the fact that if a PortFast-enabled port receives a BPDU, it loses its PortFast status and becomes a normal non-edge switchport. However, by that time, such a strong broadcast storm may ensue that the CPU of the switches may get totally overloaded and the network may be severely impacted.

A judicious use of this command is very, very much recommended.

Best regards,

Peter

Patrick McHenry · ‎07-13-2011

Thanks, Peter. That was a great explanation. That command always confused me.

Also, I never saw this response in my email. I seem to be getting them now. Have other people had the same problem?

ameen.ahmed · ‎06-13-2011

You won't be able to configure portfast on a trunk port. If you try to configure it, portfast will be enabled, but will be activated only when the port becomes an access port.

To overcome this situation, spanning-tree portfast trunk command is used. Using this command you can configure portfast even on trunk links.

I don't know any other differece or security features associated with the command.