VLAN Tagging

Unanswered Question
Mar 11th, 2008

If one end of switch is 802.1q and other end is access port.can we pass the traffic between these ports.Conceptually it should let traffic matching access port vlan to pass.Any inputs????

Also how abt frames with NULL vlan Id how is NULL Vlan Id treated???

I have this problem too.
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massimiliano.se... Tue, 03/11/2008 - 21:19


The 802.1q standard introduces the concept of "native" vlan, that is a VLAN not tagged. If you put on the other side a switch with configured access port , the frames transmitted are not tagged and belongs to native vlan.

I hope this helps.

Bets regards.


kunalbisht Tue, 03/11/2008 - 22:19

well by null vlan u mean native vlan rite,,,,

then i suppose since 802.1q doesnt tag the native vlan and the data goes untagged so i dont think the other switch will hav a problem understanding the data for native vlan,,,, as for other vlan since they r tagged i dont think they communicate unless both r trunk ports,,, plz correct me if im wrong,,,,

Istvan_Rabai Tue, 03/11/2008 - 22:30

Hi Mohammed,

When you connect a 802.1 trunk and an access port, the following happens:

The access port transmits frames untagged. The dot1q port understands untagged frames and interprets them as frames from the native vlan and forwards them on the native vlan.

If you set the native vlan on the dot1q trunk to 5, then the traffic from the access port will be forwarded to vlan 5 by the switch with dot1q trunk port.

Vlan 0 can be used to tag frames with dot1p CoS information (in quality of service configurations with IP phones), when the intention is that the frames are forwarded on the same vlan where the specific access port is located.




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