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ISL & 802.1q

Posted on 2011-02-27
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Last Modified: 2012-05-11
ISL - has some of the following which im aware of:

- Has 30 byte encapsulation
- 26 byte header
- 4 byte cyclic redundancy check or FSC
- 3 COS bit filed used for (class of service) - is this related to VOIP?

Also

802.1q

Does 802.1p - which states the following:

Made up of 4 headers
- Ethertype: 0x8100
- 802.1P: 3 COS bits used for QOS Marking - is this referring to VOIP? & as ISL is more superior than 802.1q how come QOS Markings is NOT included in ISL?
- Token ring flag: 1 bit
- Vlan id: 12 bits (1-4094 range for vlans)
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Question by:mikey250
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by:
rfc1180 earned 84 total points
ID: 34992328
>802.1P: 3 COS bits used for QOS Marking - is this referring to VOIP?
Not specifically, but for any traffic that you want to tag as priority traffic

> as ISL is more superior than 802.1q
I do not agree with that; 802.1q is non-proprietary and is the protocol that is mainly used today. ISL is a Cisco proprietary protocol that should be swept underneath the switches main board.

>how come QOS Markings is NOT included in ISL?
Sure it does:

http://www.cisco.com/en/US/products/hw/switches/ps607/products_qanda_item09186a0080116ffe.shtml
http://bradhedlund.com/2007/11/26/vlan-trunking-using-isl/http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094665.shtml

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by:kdearing
kdearing earned 250 total points
ID: 34993266
Cisco has announced discontinued suport for its proprietary ISL.
.1q is industry standard; with its biggest benefit is multi-vendor support.
You can now set up trunking between switches no matter who made them.
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by:diepes
diepes earned 166 total points
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802.1 also supports untagged traffic on native vlan, witch can be handy in some situations.
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by:mikey250
ID: 34997609
I assumed ISL was more superior than 802.1q due to frame being 'tagged' although ive read that it is an old technology, so fair enough although why, im not sure unless industry preference due to 'overhead' over ISL!
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Author Comment

by:mikey250
ID: 34997629
how can 802.1q being 'untagged' be handy in some situations, as im lead to believe due to vlan hopping and changing the native vlan 1 to say 99 or anything else is a way round to ensure protection?
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by:kdearing
kdearing earned 250 total points
ID: 34998878
Advantages of .1q over ISL

Cisco is discontinuing support for ISL - many new fixed-configutation switches don't support it
Milti-vendor support - .1q is an IEEE standard
Better performance due to header size - 4 bytes vs 30 bytes
Better performance due to native vlans - no vlan processing required
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by:diepes
diepes earned 166 total points
ID: 35000701
mikey250:
>>how can 802.1q being 'untagged' be handy in some situations,
>> as im lead to believe due to vlan hopping and changing the
>>native vlan 1 to say 99 or anything else is a way round to ensure protection?

You are correct that for security it is recommended to put the native vlan as a unused vlan, or just not use vlan1.

How is native (untagged) vlan handy ?  e.g. Want to use a linux server to sniff traffic on different vlans, but need to set it up from a remote location.
1. Change switch port to trunk with native vlan same as linux server's current subnet.  With ISL you would loose connectivity to the server/router, but not with 802.1q server will work fine on untagged vlan.
2. Now log into server/router and add vlan's as required.
This is less risky as first change is on switch and if server is not visible can be reversed before vlan's added on server/router device.
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Author Comment

by:mikey250
ID: 35010132
hi kdearingm,  are you saying 802.1q is better or other way round as 4 bytes is for 802.1q and 30 bytes is for ISL?
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Author Comment

by:mikey250
ID: 35010152
ISL can only use 'MST' for example then if vlans need to cross trunks!
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Author Comment

by:mikey250
ID: 35010252
hi diepes, "How is native (untagged) vlan handy ?  e.g. Want to use a linux server to sniff traffic on different vlans, but need to set it up from a remote location." - Just to clarify my reading, are you saying if 802.1q is used and obviously untagged, specifically on a linux server to sniff traffice then this makes it handy?

1. Change switch port to trunk with native vlan same as linux server's current subnet. - Yes im aware switching the remote connection to 'trunk' allows this!!!

"With ISL you would loose connectivity to the server/router, but not with 802.1q server will work fine on untagged vlan." - I did not realise ISL would loose connectivity but it does makes sense because traffice is secure and protected and not left open due to header info.
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Author Comment

by:mikey250
ID: 35010264
hi diepes, "This is less risky as first change is on switch and if server is not visible can be reversed before vlan's added on server/router device."  - I did not get this statement?
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by:kdearing
kdearing earned 250 total points
ID: 35032860
"are you saying 802.1q is better or other way round as 4 bytes is for 802.1q and 30 bytes is for ISL? "

Yes, 802.1q is better for all the reasons listed above.
In another few years, no Cisco devices will support it.
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Author Comment

by:mikey250
ID: 35035130
i suppose because im comparing 4 bytes is less than 30 bytes that 30 bytes was better but i accept but did not know 802.1q was better.  It was taught that ISL is protected but dont remember them saying it was better although more than likely they probably said 802.1q was preferred i suppose!! thanks for advice!!
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