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Need help with Native Vlan

CCNA, CCDA, CCNP, CCDP, CCIP, CCVP and CCIE

Need help with Native Vlan

Postby Guest » Thu Dec 06, 2007 2:20 pm

hello, with packet tracer i build the topology:

 

PC1<<<<>>>>Native 1Switch(Native 5)<<<<<>>>>(Native 5)Switch(Native 1)<<<<<>>>>>Router

 

The Pc connects to swich with vlan 1 and the switch connects to other switch with native vlan 5(on both ends). And then the second switch connects to the router with native vlan 1. I don understand why its not working ?

 

thanks for any help !

Guest
 

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Re:Need help with Native Vlan

Postby Guest » Thu Dec 06, 2007 2:22 pm

There are a couple of things about your environment that are not clear to me and which could impact the issue.

- are the switches just layer 2 switches or are they layer2/layer3?

- when you indicate that the PC1 is connected to the switch, is that via an access port on the switch or via a trunk port on the switch? When you indicate that VLAN 1 is native it implies that it is a trunk. But how does the PC recognize the trunk?

- you are not clear whether the switch to switch link is a trunk. From the indication of native VLAN it impliles that it is a trunk, but this is not clearly indicated.

- you do not indicate if the switch to switch connection is a trunk what VLANs are carried over the trunk. You indicate that VLAN 5 is native, but do not state whether VLAN 1 is also carried over that trunk.

- you indicate that the switch connects to the router, but it is not clear whether this connection is an access port or a trunk port. And if it is a trunk port you do not indicate what VLANs are carried on the trunk.

- there is a possibility that there is a configuration error on the PC. You do not tell us how the PC is configured and that might be part of the issue.

- there is a possibility that there is a configuration error on the router. You do not tell us how the router is configured and that might be part of the issue.

 

If you can clarify these items we might be able to give you better answers.

 

HTH

 

Rick

Guest
 

Re:Need help with Native Vlan

Postby Guest » Thu Dec 06, 2007 3:20 pm

sorry, for not clear question.

 

PC1<<<<>>>>(Native 1)Switch(Native 5)<<<<<>>>>(Native 5)Switch(Native 1)<<<<<>>>>>Router.

 

The PC1 connects to the switch via Access Port using native vlan 1, the other side of the switch connects to another switch using trunk links and only one vlan is allowed - its native vlan 5. The second switch connects to router using native vlan 1.

 

Just trying this topology to fully understand native vlan concept.

Guest
 

Re:Need help with Native Vlan

Postby Guest » Thu Dec 06, 2007 4:53 pm

Ok... native vlan is "only" one of vlans on trunk port. Native vlan does not have TAG (it is completely without dot1Q header, so normal ethernet frame).

If there will be some untaged frame on trunk link, switch need to know to which VLAN this frame belongs, so one vlan is assigned as native and used without header.

If you configure vlan 5 as native vlan, then frames from vlan 5 will be send thru interface without tag. If you allow only vlan 5 on interface, only vlan 5 packets will be allowed to use this trunk.

 

If you change native vlan, Vlan 1 frames are send tagged (but only if they are alowed to be send thru interface).

 

Is it clear?

Guest
 

Re:Need help with Native Vlan

Postby Guest » Thu Dec 06, 2007 5:29 pm

I will take a slightly different approach from Tomas in explaining the issue.

 

First it may be helpful to review a fundamental concept of VLAN that a frame can get from one VLAN to a different VLAN only by going through a layer 3 device that is routing between the VLANs. A layer 2 switch can not take a frame from VLAN 1 and send it over VLAN 5.

 

So in your test the frame from the PC got to first switch on VLAN 1. First switch connects to second switch by a trunk that does not allow VLAN 1. So the frame from the PC can not be forwarded to the second switch.

 

So your experiment was intended to investigate the effects of Native VLAN and wound up proving a point about VLAN forwarding.

 

HTH

 

Rick

Guest
 



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