802.1q native vlan/router on a stick.

[labsonline]

Hello, Group. Maybe someone here can help me out.

What exactly is the native vlan used for?

I've done practice scenarios where the native vlan is required to be specified and others where this is not needed. Both ways work. So what exactly is the purpose of the native vlan in 802.1q?

For example, one setup would be where a router is routing for vlans 45 and 34 and vlan 45 is configured as the native vlan on the router sub-interface and the switchport. What is the difference between vlan 45 and vlan 34 in this case? What traffic, if any, is sent on vlan 45 that is not sent on vlan 34?

Using the same scenario, I specify no native vlan on either interface and traffic is still routed by the router for both VLANs.

I tried to find an RFC for 802.1q and no luck. Cisco doc cd doesn't say much about this either.
Thanks

 

[Cluebird]

Dot1q is defined by the IEEE so you can go there to read about trunking. The native vlan is used to carry traffic without any tagging across a trunk link. Usually this vlan is a management vlan, but what it usually carries is traffic (frames) that need to be unchanged so they can be recognized by the receiving process. This can be something like a snap frame used for cdp or BPDUs for spanning-tree. The native vlan can roughly be considered a vlan for the overhead that various devices need to communicate to maintain network structure and integrity.

The tagging process is used to separate user data between the broadcast domains created by vlans.

 

[labsonline]

Thanks, Cluebird. That's pretty much what I was able to find out by researching the ieee site.

robert.