Native Vlan

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Oct 28th, 2007
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Hi all


802.1q support native vlan and i know that frames belong to the native vlan are transmitted untagged.


1. May i know what the reason to transmit frame untag ?


2. The default native vlan is vlan 1 , under wat situation will will change the default vlan?


Please help


Alan

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JORGE RODRIGUEZ Sun, 10/28/2007 - 14:50
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you have the untanged concept correct, frame tagging is used by trunking to to identify vlans that passes through that trunk link, a unique identifyer is placed in the frame header to determined which vlan the frame belongs to e.g Vlan ID2, VLAN ID 3 etc.. except for native vlan frames becased frames that come from native vlan1 do not need to be identified.


on your second defualt vlan 1 question , here is a link explaining situations in changing default vlan 1.


Refer to "Precautions for the Use of default VLAN 1" section.


http://www.cisco.com/en/US/products/hw/switches/ps708/products_white_paper09186a008013159f.shtml#wp38986


HTH

Jorge



Jon Marshall Mon, 10/29/2007 - 01:07
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Hi Alan


In addtion to Jorge's reply just to clarify on the first point.


The reason you need to be able to transmit frames that are untagged is for compatability with devices that do not understand 802.1Q tagged frames.


HTH


Jon

anand.dahiya Tue, 10/30/2007 - 09:00
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Hi Alan


I m not sure y frames are untag frames send for native vlan.


But you can change the native vlan with simple command that is


switchport trunk native vlan

bvsnarayana03 Tue, 10/30/2007 - 10:49
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In the last few few days this question has come across on this forum a lot of times. The below explanation has been appreciated by many. Also go thru the link for better understanding.


By default native VLAN is VLAN 1, but can be changed to any No. on the trunk port by command "switchport trunk native vlan #". This will make a new vlan# as native & allow all pkts from this vlan to pass thru trunk untagged.


Native VLANs are used to carry CDP, PAgP & VTP messages. Thus the Frames on native VLAN are untagged. For these messages to propagate between devices, native VLANS must match on both sides of the trunk. In case of native VLAN mismatch on bothsides of the trunk, STP will put the trunk port in err-disabled state.


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

Francois Tallet Tue, 10/30/2007 - 13:02
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IEEE control protocol generally exchange messages using untagged frame on the wire. That does not really correspond to an "untagged" vlan (in the IEEE spec, several vlans can be sent untagged on a 1q trunk. The "native vlan" is a Cisco stuff;-), but rather comes from the fact that devices using those control protocols don't have to be vlan aware. The protocol run below the layer implementing vlans if you want.

Before 802.1Q, Cisco had its proprietary trunking mechanism called ISL. In Cisco's initial implementation, vlan 1 was always allowed on all the trunks and could not be removed. That's why all the Cisco control protocols were running on vlan 1.

So eventually, when everything was moved to 1q trunks, those Cisco protocols kept using vlan 1. All that to say that if you change the native vlan of a 1q trunk to, say 23, PagP, DTP or VTP will still run on vlan 1 and will have their frames tagged!

On the other hand, IEEE protocols like STP or at least MST (Cisco's PVST is yet another deviation from the standard), LACP, LLDP etc... will still be sent untagged on the wire. So practically, it's as if they were now sent on vlan 23, "the native vlan".

regards,

Francois

bvsnarayana03 Tue, 10/30/2007 - 13:27
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Thanks Francois for the correction.



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