ISL and 892.1q ?? When to use?

mattsthe2

Every time i have setup a drunk ive always used dot1q, when would i use ISL ?

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Comments

r_durant

How do you setup a drunk? Did you give him too much drinks?

Just kidding...help me here guys...
I think ISL and dot1q runs on the 2900 series, but only dot1q runs on the newer 2950 models and up...

And i also think that if you are going to create a trunk between a 2900 and a 2950 you must use dot1q..

But as I said, I need some help here..someone to correct me if i'm wrong

spiker

maybe some of the more experianced guys on here can answer in more detail but the heart of the matter is this:

dot1q is the industry standard as layed out by the ieee (hence dot1q, it comes from ieee 802.1q) and should be used whenever you connect devices from multiple vendors, ISL is cisco's proprietary version of it, although cisco came up with theirs first.

So if your working with only cisco kit, use ISL, if not use dot1q.

hope thats what your after

r_durant

Here's some info from Cisco's site that might help...

http://www.cisco.com/en/US/tech/tk389/tk213/technologies_configuration_example09186a0080094bc5.shtml

Slowhand

Yup, that's the basic difference. ISL is Cisco-proprietary, (and usually easier to set up on Cisco devices,) and dot1q is cross-platform. Cisco generally recommends you use ISL if your environment is Cisco-only, and to set up dot1q if you're using or planning to use non-Cisco devices in your network. It's kind of the same deal as EIRGP and OSPF. EIGRP is an easier beast to deal with, but it's Cisco-only. OSPF is. . . painful. . . but it's an open standard that lots of vendors support.

And setting up a drunk is easy. Just line up the filled shot-glasses on the bartop, and put wheels on his chair. . .

Cessation

So is it safe to say that dot1q = older version of IEEE 802.1Q?
My testout doesnt go over much dot1q.

Ever been to dot1q.org?

mattsthe2

ok guys. Ha Ha very funny!!! I really want to be drunk right now ive got my test saturday so im going nuts on the last stretch.

Thanks for all the posts though.

Hmmm i had a CCIE configure all our switches and he used .1q which doesnt make sense really seeing as we are all a cisco shop.

Mind you we connect to an MPLS network so many that has something to do with it...who knows.

Expect to see many more posts from me hopefully without typo's!!!

Netstudent

Maybe he was just trying to create more interoperability with a more standardized technology or conserve bandwidth. Also ISL creates more overhead because it adds 30bytes to every frame and it fully encapsulates all frames. With ISL there is no native VLAN like there is with dot1q.

So is it safe to say that dot1q = older version of IEEE 802.1Q?
My testout doesnt go over much dot1q.

I wouldn;t say that dot1q= an older version of 802.1Q per say. I mean dot1q is just the command line reference for the 802.1Q protocol. The IOS version and featureset would probably constitute how modern the version of the protocol is.

mattsthe2

alot of trunking articles i have seen use .1q

I'm betting that is used more than isl.

This would be a good poll!!

remyforbes777

It may also be nice to include that it's not just that ISL is Cisco proprietary and .1q is a IEEE standard, ISL does not alter the frame at all but it encapsulates the frame with a 26 byte header and a 4 byte FCS where as .1q alters the actual frame.

Netstudent

and the original FCS field is recalculated.

networker050184

To tell you the truth after about six years of network experience I have never seen anyone use ISL in a live network weather its all Cisco or not. 802.1q is the norm and you should focus most of your study on it.

mattsthe2

remyforbes777 wrote:

It may also be nice to include that it's not just that ISL is Cisco proprietary and .1q is a IEEE standard, ISL does not alter the frame at all but it encapsulates the frame with a 26 byte header and a 4 byte FCS where as .1q alters the actual frame.

So its safe to say that .1q has more overhead that ISL but is compatible more yet used more...

Interesting topic.

kafifi13

Depends on your equipment as well. If you have a 2950 you can only use dot1q. So you will have to configure your other switches (for example 2924) to use dot1q as it uses both ISL and dot1q. That's how i have mine setup. I have 1 2950's and 2 2924's so the 2950 is my VTP server setup for dot1q trunking to the other two switches.

mattsthe2

ahh ok. So the old school cisco guys might typically side with .1q as it was available in the other switches.

Also sites might have a mixture of old and new switches and to save the hassle it might just be easier to setup .1q and not have to worry about it.

Cool... this site rocks!!

Netstudent

mattsthe2 wrote:

remyforbes777 wrote:

It may also be nice to include that it's not just that ISL is Cisco proprietary and .1q is a IEEE standard, ISL does not alter the frame at all but it encapsulates the frame with a 26 byte header and a 4 byte FCS where as .1q alters the actual frame.

So its safe to say that .1q has more overhead that ISL but is compatible more yet used more...

Interesting topic.

No it isn;t safe to say that. ISL has way more overhead than dot1q as I stated previously in my other post. ISL adds 30 bytes to every frame, dot1q adds 4 bytes to every frame. ISL fully encaps all frames, dot1q does not fully encap any frames.

mikearama

Yeah, what netstudent said.

I ran this by my senior admin, to get a feel for her choice to go with dot1q. As mentioned, she liked the fact that dot1q only added 4 bytes to the total packet... not 30. Apparently she used ISL in the building phase of our network, and found that several of her devices were dropping packets, since the additional 30 bytes made the packets into giants (well over 1500 mtu). Discard errors were abundant. They disappeared when she switched to dot1q.

Also, remember that dot1q doesn't add anything to the native vlan, whereas ISL encapsulates all traffic, removing the concept of having a native vlan.

Mike

dtlokee

Cisco recommends you use 802.1q for your trunking. About 6 years ago ISL encapsulation could be done in hardware and dot1q was not, it required additional processing. At that time ISL was the better choice.

One big shift is that many of the new switch models use off the shelf components. Open up a 2960 and you find a big fat broadcom ASIC inside of it. That may explain why they only support dot1q. In the future you may see more and more of the Cisco switches move to only supporting dot1q encapsulation.