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Native VLAN Discussion - Prunning and what is and isn't tagged (Control Protocol)

So the last few hours I have been reading about the native VLAN on Cisco Switches and have found allot of contradicting \ misinformed opinions on the internet.  

First of all  Clause 9 of the 1998 802.1Q standard defines

Frames belonging to the native VLAN do NOT carry VLAN tags when sent over the trunk. Conversely, if an untagged frame is received on a trunk port, the frame is associated with the Native VLAN for this port.

I'm sure that I had read control protocol information was untagged http://www.cisco.com/en/US/products/hw/switches/ps4324/products_tech_note09186a0080094713.shtml tells me differently.

So I guess it would make sense to prune both the native vlan and Vlan 1. Since pruning (switchport trunk allowed) won't affect the control protocol information.

http://blog.ine.com/2008/07/17/pvst-explained/ also tells me when useing PVST STP the switch sends the BPDU both tagged and un tagged.

This link https://learningnetwork.cisco.com/thread/7582 also had what seemed good information.

Could you please confirm that I have read is correct?

Also does the native vlan have to exist in the VTP Domain? I think not

I think the biggest mistake I made was assuming all control information was untagged

Or from a different sourse this is what I belive

STP and DTP frames have no relation to VLAN, so are always transmited over Native VLAN unless using pvst+ 

http://blog.ine.com/2008/07/17/pvst-explained/

CDP/VTP/PAgP/UDLD are always transmited over VLAN 1, if Native VLAN is 1 then will be transmited in untagged form, if VLAN 1 is tagged (Native VLAN is other VLAN then 1), protocols will be tagged with 1. 

http://www.cisco.com/en/US/products/hw/switches/ps4324/products_tech_note09186a0080094713.shtml

tells us that even if the VLAN is pruned the traffic for these protocols will still flow.

Thanks.

1 REPLY

Re: Native VLAN Discussion - Prunning and what is and isn't tagg

All what you read  is correct indeed. There is no contraddiction.

I just suggest to read the difference between ISL and Dot1Q encapsulation on ine.com to better understand the role of a native VLAN.

Just one thing more. DTP and STP have no direct relation to the vlans... not really. the implementation of these protocols can strongly affect the propagation and the extension of the vlan domains. So , although their implementation is independent, they need indeed each other

HTH

Alessio

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