When does 802.1q use a VLAN header?

I'm taking the Boson exam right now and I just realized that 802.q1 does not use a VLAN header if the native VLAN is other than 1. I thought, for some reason, that anything outside of native VLAN 1 (which is default), would require a VLAN header. Here's what I'm looking at.

http://dl.dropbox.com/u/2526790/Capture.PNG (for bigger image)

Can anyone please explain to me when 802.1q uses a VLAN header? The question basically wants to know which of the following are true based on the configuration displayed.

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Comments

QHalo

802.1q uses a VLAN header for any VLAN tagged with anything but the Native VLAN. In this case, VLAN 3 is set as the native VLAN, thus no header.
HTH

altjx

QHalo wrote: »

802.1q uses a VLAN header for any VLAN tagged with anything but the Native VLAN. In this case, VLAN 3 is set as the native VLAN, thus no header.

HTH

Gotcha, so if two switches have native vlan 1 configured, but using dot1q, and sends traffic over to a host in another VLAN, it tags the frame with the header. To VLAN 1 hosts, it doesn't. right?

QHalo

Yup. Native VLAN is never tagged. Just remember that.

altjx

QHalo wrote: »

Yup. Native VLAN is never tagged. Just remember that.

Sounds good. Thanks!

thedrama

QHalo wrote: »

802.1q uses a VLAN header for any VLAN tagged with anything but the Native VLAN. In this case, VLAN 3 is set as the native VLAN, thus no header.
HTH

Not just 802.1q but also ISL can be used for this. This depends on your brand. If at least one of them is non-cisco, you should use 1q for frame tagging.

Why do we need these? Because, inter-switch VLAN communication occurs by this way. A host assigned to a VLAN other than VLAN1
wanna talk to the same VLAN on another switch. In this case, how switch determines to send the data to the correct VLAN is
defined as frame tagging.

advanex1

ISL encapsulates everything, native or not.

thedrama wrote: »

Not just 802.1q but also ISL can be used for this. This depends on your brand. If at least one of them is non-cisco, you should use 1q for frame tagging.

Why do we need these? Because, inter-switch VLAN communication occurs by this way. A host assigned to a VLAN other than VLAN1
wanna talk to the same VLAN on another switch. In this case, how switch determines to send the data to the correct VLAN is
defined as frame tagging.

Heero

Also, there is an option to turn on tagging of the native vlan when using dot1q.

SharkDiver

In the CCNA Security materials, it is suggested that you should set the Native VLAN to an unused VLAN so that all packets are tagged. This prevents VLAN Hopping.

drkat

Oh vlans... one of the more interesting parts