Native Vlan encapsulation

Answered Question
Jul 2nd, 2007

Hi

I've got a trunk link into which I've attached a pc. The pc's data is channeled via the native vlan, my question is how can I put dot1q encapsulation on the packets. I understand by native it remains untagged but is it still possible to tag it?

Thanks

Dan

I have this problem too.
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Correct Answer by mheusing about 9 years 5 months ago

Hi,

Just a small note, with a program like Ethereal aka Wireshark, your laptop does not need to support dot1Q encapsulation in order to record such frames.

What you could do:

1) Use a FE Hub and connect switches and laptop to it. You should be able to record any switch to switch traffic. Obviously anything above FE will not work.

2) Use a SPAN port to monitor traffic from a switch. This might or might not allow you to get the desired information depending on switch model and IOS version.

I have readily used option 1) to either understand protocols and features or to troubleshoot problems. The latter was done already quite some time back, as today most switch networks are on GE trunks and there are no hubs at those speeds.

Finally to answer your previous questions:

If the ping in your scenario above will be dot1Q tagged or not on the trunk, depends on the native VLAN for the trunk. The native VLAN by default will be 1 and thus: YES the data will be dot1Q tagged. But if you reconfigure the trunk to a native VLAN of 20, data will not be tagged.

Yes, you are right, this would create the same frames like on two laptops using dot1Q tagging.

Hope this helps!

Regards, Martin

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Jon Marshall Mon, 07/02/2007 - 03:56

Hi Dan

If i understand correctly you want the PC to send tagged packets ?. If so you need to go to the NIC manufacturer's website and look for 802.1q driver support on their NIC.

If this isn't what you want please come back.

HTH

Jon

dan_track Mon, 07/02/2007 - 04:02

Hi

Thanks for your reply. I guess that's what I'll need to do. I'm trying to do some debugging from my laptop on an issue which I think is caused by dot1q encapsulation. How would you recommend I put dot1q encapsulation on my laptop traffic. Is there some way of doing this if I set the laptop port to access and somehow through the switch tell it to tag it as dot1q. I'm using 3560 switches.

Thanks in advance

Dan

Jon Marshall Mon, 07/02/2007 - 04:12

Dan

If you do want to send dot1q traffic from your laptop you are going to have to have driver support for it on your NIC. It may already have it but you need to check on the vendor website.

Jon

dan_track Mon, 07/02/2007 - 05:16

Hi Jon,

I checked and sadly they don't, mines a sony vaio. Can you tell me if the following is ok.

Let's say I have switch 1 and switch 2, on each I create a vlan interface i.e

switch 1:

vlan 20

ip address 10.4.2.1 255.255.255.0

Switch 2:

vlan 20

ip address 10.4.2.2 255.255.255.0

I then I connect the two switch via a trunk link.

If telnet to switch 1 then ping switch 2 will my data be dot1q tagged? would this be similar to getting two laptops up and running that support dot1q via their drivers?

Thanks

Dan

Jon Marshall Mon, 07/02/2007 - 05:19

Dan

It depends on what you native vlan is of course but as long as it isn't vlan 20 then yes your traffic should be tagged as it goes across the trunk.

Jon

Correct Answer
mheusing Mon, 07/02/2007 - 05:30

Hi,

Just a small note, with a program like Ethereal aka Wireshark, your laptop does not need to support dot1Q encapsulation in order to record such frames.

What you could do:

1) Use a FE Hub and connect switches and laptop to it. You should be able to record any switch to switch traffic. Obviously anything above FE will not work.

2) Use a SPAN port to monitor traffic from a switch. This might or might not allow you to get the desired information depending on switch model and IOS version.

I have readily used option 1) to either understand protocols and features or to troubleshoot problems. The latter was done already quite some time back, as today most switch networks are on GE trunks and there are no hubs at those speeds.

Finally to answer your previous questions:

If the ping in your scenario above will be dot1Q tagged or not on the trunk, depends on the native VLAN for the trunk. The native VLAN by default will be 1 and thus: YES the data will be dot1Q tagged. But if you reconfigure the trunk to a native VLAN of 20, data will not be tagged.

Yes, you are right, this would create the same frames like on two laptops using dot1Q tagging.

Hope this helps!

Regards, Martin

dan_track Mon, 07/02/2007 - 06:15

Hi

Thanks that was really useful.

One question why would the default vlan (1) i.e in my case the native vlan be tagged, but if I configure vlan 20 to be the native vlan it won't be tagged dot1q?

Thanks

Dan

Jon Marshall Mon, 07/02/2007 - 06:36

Hi Dan

I don't think thats what Martin meant. I think he meant that in your example if the native vlan was vlan 1 then your vlan 20 traffic would be tagged.

Jon

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