port configured as trunk and access

[Kevin Melton]

Forum,

I was working at a client site today and I came across what looks to me as a very unusual configuration on a switch port.

Typically we do not see switch ports which are configured both as a trunk port and an access port.

In all actuality, I would say this is an illegal configuration, but the switch allows it.  See the configuration below:

interface GigabitEthernet1/0/2
 description LA WiFi
 switchport access vlan 34
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 34
 switchport mode trunk
 speed 100
 duplex full
 spanning-tree portfast

I am not sure exactly how this configuration would even work.  I brought it to the customers attention, and the customer response was that it functions as an access port.

Is this configuration legal? 

Thanks for any comments or input here...

Kevin

[Philip D'Ath]

There is no "switchport mode trunk" or "switchport mode access".  So this means it starts as an access port but can be negotiated to be a trunk port.

[Philip D'Ath · Accepted solution]

*** CORRECTION ***

This is hard coded as a trunk port.  "switchport access vlan 34" does nothing.  However the native VLAN is set to 34, so untagged packets will get dropped into this VLAN.

Also "spanning-tree portfast" will do nothing.

[devils_advocate]

Yeah I have seen this before, its not 'standard' as such but works fine.

As Philip says, the port will be functioning as a Trunk with the Native Vlan set to 34.

As it seems to be connected to an AP (I think), the AP will likely have an IP address in Vlan34 for management and will be tagging packets for whatever SSID's its configured with (assuming its flexconnect).

Just get the customer to remove the 'switchport access vlan 34' bit as it serves no purpose all the time the port is configured statically as a trunk.