ISL and 802.1q difference?

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Jan 6, 2007, 10:42:21 AM1/6/07
Hello-I am just finishing off chapter on Trunking and wondered what the
difference was between the 2 encapsulations-in the book I am reading it
says that dot1q uses frame tagging-whereas ISL encapsulates the
frame,putting a header in to ID the frame,
My question is whether there is an advantage of using one over the
At my college the only switches we have are 2950 ,which dont support
ISL anyway.


Jan 6, 2007, 11:59:55 AM1/6/07

"daytime" <> wrote in message
The short answer is that ISL is a pre-standard cisco proprietary protocol
whereas 802.1q is an IEEE standard. Cisco now reccomends moving
away from ISL to 802.1q. So, study ISL for the exams, if you must, but
use .1q on the job.

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Jan 6, 2007, 11:37:11 AM1/6/07

Thanks for that-so all newer Cisco products support dot1q?


Jan 6, 2007, 11:38:37 AM1/6/07


Just a question.
I think; if I have got a good memory; ISL use more processor cyscle than
802.1q because of the modification of the header and "racalculation" of
the FCS. Whereas 802.1q just encapsulate and the FCS is not moficated.

I am wrong ?

Gabriele Beltrame

Jan 6, 2007, 12:27:10 PM1/6/07

You are right.

I add that while 802.1q has the concept of "native LAN", ISL do not.
It is also possible to "double tag" with 802.1q to make what's called QinQ;
I don't think a frame can be enveloped twice with ISL.


"Sven" <> ha scritto nel messaggio

John Agosta

Jan 6, 2007, 2:32:41 PM1/6/07

"Gabriele Beltrame" <> wrote in message

Dot1q is the way to go, as it is an industry standard. ISL is not a
standard, and it is dead in the market.

This is what ISL looks like:

The original ethernet frame (including the original FCS check) is
encapsulated with a 26 byte header,
and a new 4 byte FCS as a trailer. The VLAN identification from which the
original frame is associated with
is found within 10 bits of the 26 byte header, so ISL had the ability to
identify vlans 1 thru 1023.

Dot1q does not use an encapsulation technique. Dot1q uses "tagging," where a
4 byte field is inserted into the
original frame immediately after the MAC addresses before traversing the
Because the frame has now been modified by inserting additional bits, the
FCS must be re-computed and re-written.
Dot1q has a 12 bit field for VLAN identification, allowing for additional
VLAN numbering (1-4095).



Jan 6, 2007, 5:08:20 PM1/6/07

Thankyou very much for all who have more than answered my
question-could you please explain what QinQ and "native LAN" mean
Many thanks

Jan 6, 2007, 6:00:12 PM1/6/07
"Drake" <> wrote in message

Cisco is still using ISL "for internal purposes". For example, if you have
Catalyst 6500 Supervisor 2 in "Native Mode" (i.e. you have Sup CatOS module
and IOS MSFC module), the trunking protocol between Sup and MSFC is ISL.
Sure, both modules are proprietary Cisco hardware, but somehow Cisco prefers
ISL versus 802.1q in this scenario.

Good luck,

Cisco IP Phone PC Headset Adapters

Gabriele Beltrame

Jan 7, 2007, 6:32:13 AM1/7/07

Since an example could be better understood...
You have two switches (A and B ) with these VLAN configured ( 1,2,3 on
either Switch ) and native VLAN set by default on each switch ( number 1 )
When the switch A need to send frames to switch B owned by VLAN id 2 and 3
it tags them and send them on the trunk.
Instead traffic origination from VLAN 1 will not be tagged and will be sent
as is over the trunk.

Now you configure switch B trunk to consider "native" traffic for VLAN 3:

When switch A sends untagged traffic for VLAN 1 ( which it considers
native ) over the trunk, switch B considers this untagged traffic as native
but it will tag it as a member of VLAN 3 ( which switchB considers native ).
The same thing happens when switch A sends traffic tagged as VLAN 3.
So switch B considers native either the ingress trunk traffic which is
untagged or tagged with VLAN id 3.
When switch B sends VLAN id 3 traffic over the trunk, since it considers it
native, it sends it untagged.

I know, I made a mess and maybe I'll not be understood but I hope you'll get
it anyway.

QinQ ( 802.1q tunneling ) is prevalently used when deploying MPLS VPLS.
Simply put a frame is double tagged ( tagged twice ).
Dest MAC | Source MAC | EtherType | tag | EtherType | tag | EtherType | Data


"daytime" <> ha scritto nel messaggio


Jan 8, 2007, 2:11:30 PM1/8/07

So ,by double tagging each VLAN will know the correct NATIVE VLAN?-from
your example Native can be changed? I thought it was always Vlan 1 .
Thanks for the help.

Jan 18, 2007, 12:18:58 PM1/18/07
Yeah, you can change the native vlan, see below:

Specify the native VLAN:

For switches running 802.1Q as the trunking mechanism, the native VLAN
of each port on the trunk must match. By default all COS ports are in
VLAN 1; and the native VLAN on the IOS devices is also configured for
VLAN 1, so the native VLAN does match. If you choose to change the
native VLAN, use the set vlan command for COS switches or the
switchport trunk native vlan command for IOS switches to specify the
native VLAN. Remember that the native VLAN must match on both sides of
the trunk link for 802.1Q; otherwise the link will not work. If there
is a native VLAN mismatch, Spanning Tree Protocol (STP) places the port
in a port VLAN ID (PVID) inconsistent state and will not forward on the

My question is this, In what scenario would you need to change the
native VLAN?

> > "daytime" <> ha scritto nel messaggio

> >
> > > Thankyou very much for all who have more than answered my
> > > question-could you please explain what QinQ and "native LAN" mean
> > > please?

> > > Many thanksSo ,by double tagging each VLAN will know the correct NATIVE VLAN?-from

> your example Native can be changed? I thought it was always Vlan 1 .

> Thanks for the help.- Hide quoted text -- Show quoted text -


Jan 18, 2007, 4:56:34 PM1/18/07

Thanks for the reply-I too have no idea why you would change the native
>From what I read in the above posts,it is to do with when the VTP
server propogates information to other Client switches in the same VTP
domain,and that could cause problems as each Native Vlan on each
individual switch will have different information...?


Jun 25, 2016, 11:38:04 AM6/25/16
replying to daytime, bksun wrote:
*ISL is a Cisco proprietary protocol* for the interconnection of multiple
switches and maintenance of VLAN information as traffic goes between switches.
In ISL, the original frame is encapsulated and an additional header is added
before the frame is carried over a trunk link.
First ISL came that works only for cisco devices later *. ISL is supported in
Cisco 1900 series switches only. But 802.1q is su
pported in cisco 2900 and above series switches.

In 802.1Q(IEEE standard) 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.switch

_*ISL support only cisco switches but 802.1q support cisco and non cisco
switches both.*___

Hope it is clear..

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