VLANs – Tagging “ALL” Traffic Implies ISL

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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 Cisco.com 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 is a podcaster and writer with a BSCS and 20+ years in enterprise IT. He's operated data centers with a special focus on infrastructure — especially networking. He's been a CNE, MCSE, CEH, CCNA, CCNP, CCSP, and CCIE R&S #20655. He's the co-founder of Packet Pushers Interactive, LLC where he creates content for humans in the hot aisle.

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