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untagged trunk

I was able to perform this on an Avaya switch, but we're moving to Cisco and I'm not sure how to perform this function on a CISCO. I'm aiming to do this on a C9200L



device interface -vlan 32,40, with PVID 40. All untagged.


It requires both to be untagged, as the device has errors when it is tagged. It needs to be on both networks, as it needs to receive broadcast traffic from both, while broadcasting it's own traffic on the PVID. It must do this on a single network interface.


On avaya I could set "flexible tagging mode" and set multiple untagged vlans on the port.


I know this is a very non-standard configuration, but the equipment is over 20 years old and is very specific to it's requirements.

VIP Expert

Cisco only allow 1 Native VLAN example  should work for you  as per the description, do test and let us know.


interface GigabitEthernet x/x
switchport trunk native vlan 40
switchport mode trunk


if you want more control :


interface GigabitEthernet x/x

switchport mode trunk
switchport trunk native vlan 40
switchport mode trunk allowed vlan 32,40



*** Rate All Helpful Responses ***

Hi Balaji,


You might have missed the part in my post where all the vlans must be untagged, not just the pvid.


I've tested this as a regular trunk port as you've suggested without success.


VIP Advisor


 - Strange also in terms of standards, device should be vlan-aware if  it does not want to diffuse traffic on unneeded vlan's , opinions in these threads seem to align :


The device is for DECNET and VAX communication, so it's very particular. It's not VLAN aware, and needs to communicate to both.


The DECNET and VAX systems need to be on separate VLAN's as they are very sensitive to noise.

I can get this to work on nortel/baystack/avaya/extreme switches as they allow you to have multiple access vlans on a single port ( or fully untagged trunk). I'm hoping that Cisco can provide some sort of similar funcionality.



 - Must use some mac address prefix knowledge to know to whom to talk too. Must defend cisco here, as other reply indicated , that is very outdated ,modern hardware such as hypervisors , fileservers , etc are all vlan-aware in these days.  I used to work on DECnet and vms too , more then 30-years ago , where are the days (smile).


I agree, all modern systems are generally vlan aware, however these aren't modern. They were installed in the 80s (VAX) and early 90s (DEC), and cannot be replaced yet.


I wish our production used hardware built in this century, but it is not. All I know is that if I change the interfaces to standard Trunk format, or add any form of tagging, the systems fail to communicate until changed back.


Joseph W. Doherty
Hall of Fame Expert

Interesting that other vendor(s) allow this, because consider where should ingress traffic on such a port be directed?  I.e. you effectively changed multiple VLANs into one VLAN.

Two ways I see it might be done on a Cisco device.  On a L2 switch, you might have one all three VLANs directed to access ports, one for each VLAN.  Then you might also have additional access ports for one of the VLANs.  Those ports would be connected to access ports other than the one VLAN.


access port VLAN 10 (this port would should all the VLANs traffic)

access port VLAN 20 connected to a VLAN 10 access port

access port VLAN 30 connected to a VLAN 10 access port

The other way, on a L3 switch, would be to have your multiple VLANs routed to a routed port.

For the other vendors, ingress traffic is defaulted to the PVID (native vlan). The end result is that the device is listening to all the vlans on the port, and it's outbound traffic is on the PVID, without the device seeing the tags.


None of the protocols it uses is directed traffic, all the devices on these subents only communicate with a broadcast type traffic, either in form of modbus, decnet, SAF, or whatever the VAX is using.