From: Joe Mama (jsmith1234550@gmail.com)
Date: Tue May 29 2007 - 12:55:34 ART
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
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