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802.1q Native vlan

Hi,

Could anybody please explain to me what actaually the native vlan of 802.1q is? What's the purpose of its existence? When'll it be used? Thank you

3 REPLIES

Re: 802.1q Native vlan

With an 802.1q trunk link all VLANs EXCEPT the Native VLAN carry a Tag that indicates the VLAN number (as well as the 802.1p priority tag). Packets that are received on a Trunk link WITHOUT an 802.1q Header are treated as received on the VLAN ID that is the Native VLAN. Any packets sent without an 802.1q header should be sourced from the Native VLAN.

Andy

Gold

Re: 802.1q Native vlan

Andy nailed it.

Specifying a native VLAN tells the switch or router which VLAN to associate the traffic with, if the traffic that it has received has no VLAN tag.

One use would be to pre-configure a switchport as an 802.1Q VLAN trunk, but also assign a native VLAN. Then, if you plug another switch in that's configured for 802.1Q, it'll trunk; if you plug in a computer or printer that doesn't do 802.1Q tagging, then it'll act like a VLAN access port, sending and receiving on the native VLAN.

Re: 802.1q Native vlan

Hi,

just one additioninal info:

Cisco implements 802.1q with native VLAN obligatory - i.e., there HAS to be one VLAN on a 802.1q trunk chosen as a native VLAN (VLAN1 as default).

From IEEE point of view native VLAN is an option on 802.1q trunk.

So some other vendors (3Com, e.g.) allow you to configure a native VLAN, but there is no by default.

Some vendors don't implement native VLAN at all. You need to configure all VLANs on 802.1q trunk to be tagged. There is a set dot1q-all-tagged CatOS command and (but I never tested it) which should enable this feature.

Regards,

Milan

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