vlan encapsulations

Unanswered Question
Aug 14th, 2007

Hello everybody,

I have small doubt any body can help that is what is the difference in between dot.1q & 802.1q.

It would be appreciated.


I have this problem too.
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mohammedmahmoud Tue, 08/14/2007 - 03:56


ISL (Inter-Switch Link)

Cisco Proprietary

Encapsulation - new 26byte header + 4byte FCS

Protocol independent (encapsulate the frame inside a new frame - It doesn't modify the original frame)

It supports 1005 configured VLANs (new implementation supports the 4094)

In ISL any unencapsulated frames are dropped


IEEE Standard

Tagging (The Tag (4 Bytes) is added after the source and destination MACs - The FCS needs to be recalculated)

Protocol dependant (just adds a tag to the original frame - Original frame is modified)

It supports 4094 Configured VLANs.

Native VLAN not tagged.


Mohammed Mahmoud.

Richard Burts Tue, 08/14/2007 - 04:05


You gave Naidu a nice discussion of ISL and of dot.1q. And the things that you say are true. But that was not the question asked in the original post. The original question was:

difference in between dot.1q & 802.1q

and the simple answer is that there is no difference. dot.1q is just a simple way to refer to 802.1q.



guruprasadr Tue, 08/14/2007 - 04:09


My comments are, For me dot.1q & 802.1q are same.

I feel there should be no difference between these.

dot.1q is a easy way of representation for 802.1q


Best Regards,

Guru Prasad R

ilnaiduccna Tue, 08/14/2007 - 04:11

Hi Rick,

Nice diverse thank u, as u told mohmmed explained well but that is not what exactly I asked.

In ur post there is no difference in between dot.1q & 802.1q then why we are assigning dot.1q for router subinterface and 802.1q we are assigning where router subinterface using as a VLAN in Switch.

Why we should not use 802.1q encapsulation for both like Router subinterface & VLAN in switch.

Can u give small clarification on my post.

Thanks in advance,


mohammedmahmoud Tue, 08/14/2007 - 05:01

Hi Naidu,

I am extreeeeeeeeeeeeemly sorry for that :), i am sleeping less than 3 hours a day for at least a month now, and i though i've read your post asking about the difference between ISL and 802.1q, please do accept my apologies :)

Now to your real question, 802.1q is the same as dot1q, there is no difference at all, the 2 words are freely used interchangeably to describe the same thing which is 802.1q trunking (dot1q word describes the ".1q" part of the IEEE 802.1 standard naming).


Mohammed Mahmoud.

ilnaiduccna Tue, 08/14/2007 - 21:55

Hi Mohammed,

No Problem. you no need to applogie, it was a good answer, just an answer to a different question, however I learnt one new thing.

I also like to come forward and Thank both of you (Rick & Mohammed Mahmoud) for your Great Contribution to the Entire NetPro Community. I have had truely learnt a lot in your Posts.



mohammedmahmoud Wed, 08/15/2007 - 04:26

Hi Naidu,

Thank you very much for your kind words :) NetPro is indeed a nice place to discuss all your technical issues, and its really great being a part of the NetPro community.


Mohammed Mahmoud.

Richard Burts Tue, 08/14/2007 - 05:14


No Problem. It was a good answer, just an answer to a different question. I have had the same experience myself of composing an answer to a question and then to realize (sometimes in public) that I had not really understood the question.

As a consultant I find that it is very important to understand exactly what the customer is saying and to understand fully what their problem is. I attempt to apply this also to activity in NetPro.



mohammedmahmoud Tue, 08/14/2007 - 05:33


I really appreciate your words, you are totally right, having professional and respectable people like your self in NetPro really makes this place special, my fault here is that i am very exhausted between my work and my lab preparation and still i am trying to contribute in NetPro which has an obvious resultant which is suboptimal answers :), anyway wish me luck please next November.


Mohammed Mahmoud.

guruprasadr Tue, 08/14/2007 - 05:42

Hi Rick & Mohammed Mahmoud,

I also like to come forward and Thank both of you (Rick & Mohammed Mahmoud) for your Great Contribution to the Entire NetPro Community. I have really learnt a lot in your Posts.

I always expect your Wishes, Support & Guidance in my Career.

Thanks & Regards,

Guru Prasad R

cindylee27 Tue, 08/14/2007 - 21:35

Yup,agreed with Guru..:)

Rick and Mohammed, they are great guys in NetPro..Really appreciate their efforts..

And to Mohammed, good luck in Nov ya for the lab test..:)

mohammedmahmoud Tue, 08/14/2007 - 22:38


Thank you both Guru and Cindy for your kind words :) i really appreciate this appreciation.


Mohammed Mahmoud.


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