Re: DTP and Portfast together?

dc_specialist · ‎08-21-2019

I have two questions the first is:

After a port has became a trunk port is there any way to see if that port was originally set to dynamic desirable, dynamic auto, or just trunk?

I am using the sh spanning-tree detail command to identify trunk ports. Then using sh int <specific interface> switchport command to gather more info. But, I am not sure if either of these specify the port's previous state.

Switching, Other Switching

My next question is can a port be set as portfast and enabled with a trunking protocol?

As I use the sh dtp interface command and the sh spanning-tree detail I am discovering a few ports that show up as portfast enabled and listed under the dtp interface with a trunking protocol of 802.1q. Is this normal? My first thought was if a port is portfast it would go errdisable if a switch is connected, so why have any trunking protocol on the port. is my thinking wrong?

Thanks

Giuseppe Larosa · ‎08-21-2019

Hello ,

show interface trunk should show n-dot1q where n means negotiated for cases where the trunk has been formed by DTP negotiation.

At least this happens in some Catalyst switch platforms.

show interface switchport should provide the Admin state and the operation state and you should be able to discriminate between ports with manual mode trunk and ports relying on DTP desirable state.

>> As I use the sh dtp interface command and the sh spanning-tree detail I am discovering a few ports that show up as portfast enabled and listed under the dtp interface with a trunking protocol of 802.1q. Is this normal? My first thought was if a port is portfast it would go errdisable if a switch is connected, so why have any trunking protocol on the port. is my thinking wrong?

It depends on STP bpuguard settings if STP bpuguard is enabled yes the port should go in errordisable when a switch is connected to it. If STP bpdu guard is not enabled the port can accept the switch.

You can configure spanning-tree portfast on a trunk port but you get a warning message about the risks.

Hope to help

Giuseppe