trunking and native vlans

Unanswered Question
Oct 22nd, 2007

I just did a lab where the scenario was vlan information was not being propagated over a trunk link (only 2 switches in this lab).


After checking the obvious (eg trunk is operational, matching vtp domain names, etc.), it turned out that you had to issue a "no shut" command for the native vlan (using 802.1q, native vlan 1).


It seems the native vlan has some association with trunking. What exactly is the connection?

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pciaccio Mon, 10/22/2007 - 17:09

The native vlan in Dot1Q trunking is very important.The native VLAN is the only VLAN that is not tagged in a Dot1Q trunk (802.1q). In most cases the native VLAN on a trunk is VLAN 1. If you do not have the same native VLANs on both ends of your trunk then you will receive native VLAN mismatch errors on your switches. This does not mean that the trunk does not work. It does if you have the VLANs defined on your switches that are propagating through your trunk. It is good practice to not use your native VLAN for any concernable traffic. Make it a junk VLAN and have no data on it. It is not tagged in this type of trunk thus if you are connecting to a third party vendor with your trunk then you are at risk of exposing your native VLAN to them and if it has traffic on it , that too... Hope this helps..Good Luck...

hi.622823 Mon, 10/22/2007 - 17:14

Hi,


I am puzzled with native vlans not being tagged. I mean, if all the other vlans get tagged, why should the native vlan be different (isl, as we know, tags all vlans)?

Francois Tallet Mon, 10/22/2007 - 21:14

Being able to send untagged frames on a link allows you to connect devices that don't understand the 802.1Q tag. That's something that was never important for Cisco trunks (supposed to be point to point between bridges), but 802.1Q is an IEEE standard and is not a "trunking" mechanism in the way Cisco defines it.

Regards,

Francois

Nagaraja Thanthry Tue, 10/23/2007 - 04:27

While ISL was designed exclusively to connect switching devices, DOT1Q is a generic form. I guess, originally, Native VLAN was introduced to carry management traffic between switches. Not tagging these management messages reduces burden on the switches as they do not have to untag every packet to identify the management traffic.

prabhdeepnijjar Thu, 10/25/2007 - 07:18

Native Vlan do have connection with the trunking...let me take a simple scenario..

You have two switches connected together and you have vlans on both of them .but in one of the switch one the port has not been assigned to any Vlan .then that ports data will be sent to native vlan without any configuration on tht particular port.

This native vlan is best used for administarive purpose and also in VOIP configuration..

trunking is there but done only for unassigned vlan ports

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