Ethan Banks On productivity.

VLANs – Tagging “ALL” Traffic Implies ISL


In my last post, I mentioned the following:

I should have done ISL trunks, not 802.1q, 3 points. That was an issue of nasty wording. The tasks stated “All traffic sent over these logical links should be tagged with a VLAN header”. I looked at the word “tagged”, thought immediately of dot1q, and didn’t think about it again. But only ISL trunking includes a header – so “header” is the word I should have keyed off of.

Another candidate pointed out to me via e-mail that if all traffic should be tagged, then the implication is ISL.  Why?  Because 802.1q has the concept of a native VLAN, the VLAN that untagged traffic is considered to be in.  On the other hand, ISL trunks always encapsulate an ethernet frame – there is no untagged, native VLAN traffic on an ISL trunk.  Ergo, if all traffic must be tagged, then only ISL can satisfy that requirement.

Read this article for more information on the differences between ISL and 802.1q:  Inter-Switch Link and IEEE 802.1Q Frame Format.

1 comment

  • But you could also use the vlan dot1q tag native command, to tag ALL traffic with dot1q. Would that have also fullfilled the requirements? I think you were right with the “header” keyword, although the wording still leaves a lot to be desired. I hope the real lab is not as ambiguous.

By Ethan Banks
Ethan Banks On productivity.

You probably know Ethan Banks because he writes & podcasts about IT. For example, he co-authored "Computer Networks Problems & Solutions" with Russ White.

This site is Ethan on productivity--not tech so much.

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