Why native vlan is used

[Masoud Pourshabanian]

Hello everybody,

I know that native Vlan is configured on Trunk links and switch does not add Vlan ID to a frame going to or coming from a native Vlan. I also know it is used for compatibility with devices which do not support Vlan tagging. My question is why we connect those devices to a truck port. We can easily connect them to an access port. If those devices do not support Vlan tagging, connecting them to a trunk port is meaningless. Could please give me an example to clear up my confusion.

Thanks

[Ahmed Muhi · Accepted solution]

Hello M. Pourshabani,

Your question is two parts, let us answer the first part, What is a native VLAN?

As you know a switch has two types of ports, a trunk port, and an access port. On an access port the switch expects to receive traffic untagged, since end points do not understand the VLAN concept, while trunk port are expected to receive tagged traffic.

So you see when untagged traffic is received on an access port the switch would simply associates traffic from that port with VLAN configured on the port. As for Trunk ports, traffic should be tagged, and the switch use this VLAN information to decide which VLAN the traffic belongs to, this leads us to the Native VLAN concept.

If a switch is to receive untagged traffic on a trunk port what will the switch do with it, to which VLAN it will associate it, here is where the Native VLAN concept is used for untagged traffic.

Second:

Why a device with no VLAN support would be connected to a trunk port, why not an access port ?

You see Native VLAN was created when hubs where still being used, and you would normally have two switches connected to each other by a hub. Devices that are unable to tag frames might be connected to the hub and as you know hubs can not tag frames, now a switch would receive untagged frames, causing the problem that Native VLAN is trying to solve, see below image:

[stefanmtomasevic]

I still have a trouble to understand it.

To be more precise: "You see Native VLAN was created when hubs where still being used, and you would normally have two switches connected to each other by a hub. Devices that are unable to tag frames might be connected to the hub and as you know hubs can not tag frames, now a switch would receive untagged frames, causing the problem that Native VLAN is trying to solve, see below image".

Yeah, but that was in back in time when were hubs are used, and there are not in use anymore. All devices that I would connect to the modern (catalyst) switch, can be connected to access port with assigned vlan. So this is the reason that I still do not understand use of native vlan today. Are there any situation not to assigned vlan on access port for any devices?

Thank you for your time.

[Masoud Pourshabanian]

Thanks for your accurate response. My problem was with the second part of your answer which was solved.

[Ahmed Muhi]

I am glad I was able to help.

Best wishes.