Types of Ethernet Frames

FrankGuthrie

I've found there aere 4 different types of Ethernet Frame:

• Ethernet II
• IEEE 802.3
• IEEE 802.2 LLC
• SNAP

What is the difference, between these 4 frame types and which one used for what (application). Alre they all used in daily life? Why do we have 4 types and not 1 frame which can do all.

FrankGuthrie

Anyone?

networker050184

Have you checked out the Odom CCNA books? This was all covered extensively in the first couple chapters last time I read through. I don't remember the details off the top of my head though. I could google it for you if you really want me too but I'm sure you can do that yourself.

Sy Kosys

FrankGuthrie wrote: »

Anyone?

Let me google that for you

FrankGuthrie

Well I'v tried google, but it just doesn't make it clear for me.

Do we have four types Ethernet frames which are used or just 2 types ( SNAP and LLC are subs of IEEE 802.3???)?

Sy Kosys: If you don't have anything useful to say, don't bother to reply on the thread, unless your going for the most postcount by replying with useless comments.

goldenlight

I will try to answer question base on my limited knowledge

Reference taking from Cisco Netacademy

Snap: Is a field in the VTP Frame used in Vtp Advertisements
IEEE 802.3 adds the field Start Frame Delimiter, and increases the data field size to a max of 1500 bytes
Ethernet II : ??? Ethernet II is the Ethernet frame format used in TCP/IP networks.


I would have to say 802.2 standard is retired, but then again I could be wrong.


Keith barker might be the one to chat with, but man he might get a little carried away with that question..

Sy Kosys

FrankGuthrie wrote: »

Well I'v tried google, but it just doesn't make it clear for me.

Do we have four types Ethernet frames which are used or just 2 types ( SNAP and LLC are subs of IEEE 802.3???)?

Sy Kosys: If you don't have anything useful to say, don't bother to reply on the thread, unless your going for the most postcount by replying with useless comments.

Cry me a river.

Click the link i posted, second search result takes you DIRECTLY to cisco, where it actually explains it.

And yes, it IS easier to type all this than it is to copy/paste.

Give a man a fish.....

PS> Gee, looks like my post count just went up one, cheers

FrankGuthrie

Ok, I asked the question on CLN

And I got the following anser (An actuall awnser and not a snarky remark ):

There are three types,
Ethernet II
802.3 with 802.2 LLC
802.3 with 802.2 SNAP

I'm not sure why Ethernet II became most popular but it might be due to 802.3 with 802.2 LLC was a bit lacking as I described in my blog post if you need to have many different SAPs.

802.2 defines Logica Link Control (LLC). You might remember that MAC layer in OSI is actually divided into MAC and LLC. The 802.3 standard is very complex, you can read it at IEEE if you want. It describes everything that must be supporter for transceivers like auto negotiation and all the things like that.

However I did not get an awsner and my next question to get confirmation:
So to me it is now clear the 802.2 definitions are basically addition/extension to the excisting 802.3, but with a separate subnumber. Is this a correct assupmtion?

FrankGuthrie

Wondering if i was on the right track:

I've found there are 4 different types of Ethernet Frame:

• Ethernet II
• IEEE 802.3
• IEEE 802.2 LLC
• IEEE 802.2 SNAP

It seems I'm getting close to understanding this. So Basically the Ethernet II frame which was created by the DIX consortium in the most used Ethernet frame used in networks today.

The IEEE decided to create s 'open standard' made by them. They came out with the IEEE 802.3 standard and this was quickly taken over by Novell, and this frame is came to know as IEEE 802.3 RAW. Bu the IEEE decided to make change in there original draft and created both the 802.2 LLC and SNAP additions.

The only thing I think I don't get is the different between LLC and SNAP.

xnx

I suggest you read Data and Computer Communications by William Stallings

EdTheLad

Hi Frank good question, i wondered about this a while back, it took a bit of digging to figure it out.

Difference between Ethernet II and 802.3 is that Ethernet II uses the 2 byte field following the src address as a type field while 802.3 uses it as a length field. This lack of type field in 802.3 means that an interface with 802.3 can only support one type 3 encapsulation as there would be no way to differentiate between different layer 3 protocols. So 802.2(SAP,LLC) was created due to the lack of the type field in 802.3. You will pretty much always see 802.3 with a 802.2 sap header.

Now we look again at Ethernet II, it has a 16 bit field for type, which means 2^16 different layer 3 protocols can be defined. Now looking at 802.2 it has 8 bits for dsap and 8 bits for ssap, which means 2^8 different layer 3 protocols. 256 types is not enough so ieee needed to create an additional header SNAP to give the added flexibility to vendors to create there own protocol types. The SNAP header has a 3 byte Org code which is just like the 3 byte code used in the mac address, assigned to a vendor. The additional 2 byte type code gives the vendor the ability to create their own layer 3 type fields without requiring to register them. If the org code in the SNAP is all zero's, the the type field adheres to the well known ethertype values.

ISIS uses 802.3 with 802.2 SAP

FrankGuthrie

Hi Ed,

So does this mean that the IEEE made 2 additions to the frame? 1 was made before (LLC) the IEEE Ethernet frame originally came out and 1 was made after (SNAP)

So what I'm saying is that they came out with the IEEE 802.3 frame with LLC (802.2) and after they decided they needed the SNAP field, and created the IEEE 802.3 with both the IEEE 802.2 LLC field and the IEEE 802.2 SNAP field. Is this an correct assumption?

So the original finished IEEE frame already had the LLC addition to it, and the SNAP addition was the only addition made afterwards?

Just trying to wrap my head around it as I have read serveral articles but some of them state different stories and it doesn't make it 100% clear. That's why I'm asking smarter people then me

EdTheLad

I'm not sure weather IEEE made 802.3 and 802.2 at the same time or not, they probably did. But Novell deployed raw 802.3(without 802.2) for IPX. The rule was that the first 2 bytes of an IPX packet sent over ethernet must be FFFF, which made the later 802.2 distinguishable as the value FFFF in the dsap and dsap fields were unknown to 802.2. Later Novell deployed 802.2 LLC/SAP with the additional fields for type dsap/ssap. Later on IEEE realized they needed a larger type field so they stuck an additional header on the end of the 802.2 header i.e. the SNAP. A value of AAAA in the dsap and ssap tells the receiver the SNAP header is present.

So that's pretty much it, i don't think you can rate people as smarter just because they've read more than you .

FrankGuthrie

Hi Ed,

Thanks for replying. This made it 100% clear. And I like too think that people who read more are smarter then me, because the kn0w more than me. I don't like reading .

Thanks again.