From: Ryan Morris (ryan@egate.net)
Date: Tue May 29 2007 - 18:33:55 ART
There are two solutions to this problem:
1) Set the native vlan for the trunk to an unused vlan. Then every
packet that traverses the trunk (in a vlan that is used) will have to be
tagged
2) Check out this command:
3550-1(config)#vlan dot1q tag native
R.
On Tue, 29 May 2007, Joe Mama wrote:
> Hello all,
>
> Any examples and information would be greatly appreciated. I've
> looked through the archives and various books,but would still
> appreciate more clarification.
> Here's the requirement: The trunk link in should allow all VLANs to
> travel across with their VLAN ID intact. You cannot use the Cisco
> proprietary protocol to achieve this. Every packet that traverses the
> link must have the VLAN ID, no exceptions.
>
> The most likely answer is to use the following command on all 3550/60's.
>
> sw2(config)#vlan dot1q tag native
>
> Here's my basic understanding:
> Using "vlan dot1q tag native" - All traffic that is not tagged will be
> accepted and tagged in to the vative VLAN (VLAN1 by default) and go
> over the trunk port. All traffic that is already tagged will continue
> to keep its VLAN ID through the trunk. The switch accepts untagged
> packets, but sends only tagged packets.
>
>
> Without "vlan dot1q tag native", the data on the trunk will not be
> tagged but will still be on the native vlan (untagged).
>
> "802.1Q is the IEEE standard for tagging frames on a trunk and
> supports up to 4096 VLANs. In 802.1Q, the trunking device inserts a
> 4-byte tag into the original frame and recomputes the frame check
> sequence (FCS) before the device sends the frame over the trunk link.
> At the receiving end, the tag is removed and the frame is forwarded to
> the assigned VLAN. 802.1Q does not tag frames on the native VLAN."
>
> My questions:
> 1 - vlan dot1q tag native - is there another solution besides this command?
>
> 2 - Before using vlan dot1q tag native, how does the switch process
> tagged and untagged incoming and outgoing frames? My understanding is
> that dot1q encapsulation on a trunk will tag all the VLANs except the
> native
> vlan. So then vlan dot1q tag native tags even native traffic where
> normally the switch would not tag the info. - Is this correct?
>
> 3 - When is traffic dropped due to a tag not being or being present?
> What are some practical scenarios and reasons related to tagging /
> untagging? I'm trying to get a deeper understanding. I do understand
> that untagged frames can be placed in a VLAN that is more restrictive,
> etc. and do understand how having different Native VLAN in a switching
> domain can result in leakage and different results but some other
> practical examples would be nice.
>
> Thanks,
>
> Joe
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
This archive was generated by hypermail 2.1.4 : Fri Jun 01 2007 - 06:55:22 ART