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Native Vlan Questions

switchport trunk encapsulation dot1q

switchport trunk native vlan x

switchport trunk allowed vlan 1,x,y,1002-1005

switchport mode trunk

switchport voice vlan y

Taken from part of the configuration.

I have some questions about this configuration.

I read from the book that native vlan are not encapsulated with any tagging information. IEEE 802.1Q adds a 4-byte tag to the frame. This protocol can operate between equipment from multiple vendors.

1) Say from this configuration, a PC is connected to vlan x. But why the vlan must not encapsulated in order for the PC to be used ?

2) Are there any command, for the PC to use without using native but is trunk?

3) Since we are using 802.1Q and it is meant for multiple vendors, why we need it to be without encapsulation?

4) Just a general question for this. For this configuration, why we need to include the vlan 1002 to 1005? I read from the book 1002 to 1005 is a default for token ring,fddi,trnet and fddinet.

2 REPLIES
Cisco Employee

Re: Native Vlan Questions

Hi Kian,

The first and formost reason is dot1q trunking cannot be configured without native vlan (except of some new switches where you can tag native vlan also).

1) The reason pc vlan should not be eancapsulated is because when pc send out any frame it does not have any tag or vlan information and if you do not configure native vlan on trunk port then switch will not understand which vlan your frames belong to as the frame are coming into the switch without vlan information and switch will drop the frame. Now if you have native vlan switch will understand that if frame comes into its port without vlan information it belongs to native vlan.

2) Your pc can be configured to send data with tag or vlan information but not all nic cards supports tagging/dot1q tag. If your nic cards supports that you can very well configure that any frame moves out of your pc should carry that vlan tag.

It is not at all mandatory to include vlan 1002 to 1005 in your command list to allow those vlans on trunk and when you do show vlan you will see that these vlans are not supported on your platform.

HTH

Ankur

*Pls rate all helpfull post

New Member

Re: Native Vlan Questions

Hi ankbhasi,

Thanks for your explanation. I have some questions related to your answer.

1) The voice vlan does not use native and is a cisco product. Am i right to say Cisco products will understand all the vlan tag and so will not need to use native vlan?

2) Since PC need not encapsulate, Does this apply to all other equipments and servers connection? Does it use native vlan?

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