Example of using the native VLAN (IOS)?

[rigrazia]

Hi,

Does anyone have an example of how they are using the native VLAN, other than by default? In other words, does anyone have an example or a scenario where they need to send certain traffic across the trunk untagged?

CCO says, "The IEEE committee that defined 802.1Q decided that because of backward compatibility it was desirable to support the so-called native VLAN, that is to say, a VLAN that is not associated explicitly to any tag on an 802.1Q link. This VLAN is implicitly used for all the untagged traffic received on an 802.1Q capable port."

Thank you in advance.

[milan.kulik]

Hi,

one example:

Current topology:

Switch ---- PC using access port in VLAN10.

You want to add IP phone using only one port on the switch.

You can connect:

Switch --- IP phone --- PC.

You can configure the switch port as 802.1q trunk, IP phone to send 802.1q tagged frames in VLAN20, PC still sending untagged frames and with native VLAN=VLAN10 everything works fine.

Regards,

Milan

[rigrazia]

Thank you for your reply. But is there any reason why the PC needs to be sending untagged frames? I see how to configure tagged and untagged VLANs, but not clear on why you would want to send some frames, i.e. from the PC in this case, untagged.

Thanks.

[milan.kulik]

1) It's always easier to leave PC configuration default (not tagging).

2) Some legacy NIC drivers might not support frame tagging.

Regards,

Milan

[ahojmark]

The following is a working example of an access switch using the native VLAN 200 and 202 for voice:

interface FastEthernet0/1

switchport trunk encapsulation dot1q

switchport trunk native vlan 200

switchport mode trunk

switchport voice vlan 202

spanning-tree portfast

-A

[rigrazia]

Thank you for your reply. But is there any reason why one VLAN needs to be sending untagged frames? I see how to configure tagged and untagged VLANs, but not clear on why you would want to send some frames untagged. Why couldn't these same frames be sent tagged?

Thanks.

[Kerod-Napoleon]

I don't believe your question is answered? I am also struggling to understand why someone would explicitly send untagged frames!

[Joseph W. Doherty]

Because, as noted by the other posters, you might have an "old" device that cannot support tagged frames sharing a link was newer devices that can. It's not something you run into very often, but then again, in an Enterprise network of 5,000 network devices that I supported just a couple of years ago, we will had some legacy devices that would only work at 10/half. They created a problem when we installed some new switches (non-Cisco) that didn't support 10/half, although they supported 10/full. Of course, these legacy devices were critical to the business and there were no newer available replacements.

Such support of untagged VLAN and tagged VLANs on a trunk also precedes access ports that support untagged frames along with a single VLAN of tragged frames (i.e. access ports with a voice VLAN). With this support, the need to use a "trunk" with an untagged VLAN is likely much diminished.

[rwetsah]

Hi Karod, 

I will try.

In real scenarios, we have multiple data vlans such as vlan 100, 200 and so on. If the PC in vlan 100 is connecting through an IP phone, which is common, it won't be able to sent out traffic if the trunk would not be configured to sent vlan 100 frames untagged. Dont forget that the PC doesn't understand tagged frames so it would not honor tagged vlan 100 frames. I am absolutely certain if this is correct.

Cheers !