question about native vlan

mtechnology · ‎02-04-2007

I am little confused with the native vlan concept in 802.1q frame.

When a native vlan frame is said to be untagged, does it mean the ethernet frame does not carry the TAG field at all.

If this is the case, then how exactly the frame is delivered to the members of that particular vlan.

Is it when the frame finds no tag it sends out the packet to the nodes on the native vlan

thanks in advance

glen.grant · ‎02-04-2007

You only have 1 native vlan and that has to match on the other end of the trunk so the traffic which is untagged gets delivered just fine at the other end of the trunk link just like if the link was only a access link , all other vlans that are carried across that trunk are tagged so they get delivered also .

mtechnology · ‎02-04-2007

Thanks for the quick reply, but what is reason we do untag the frame.

Jon Marshall · ‎02-04-2007

Hi

It's basically for backwards compatibility. Excerpt from Cisco doc on vlan security

"On the other hand, 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.

This capability is desirable because it allows 802.1Q capable ports to talk to old 802.3 ports directly by sending and receiving untagged traffic. However, in all other cases, it may be very detrimental because packets associated with the native VLAN lose their tags, for example, their identity enforcement, as well as their Class of Service (802.1p bits) when transmitted over an 802.1Q link. "

Full link:

http://www.cisco.com/en/US/products/hw/switches/ps708/products_white_paper09186a008013159f.shtml

HTH

Jon

mtechnology · ‎02-04-2007

Hi thank u so much i got the reason