Ethernet 802.3 vs. Ethernet II Frame

[AdamBudzinski]

hi guys,

there are 2 types of Ethernet framing:  802.3 and Ethernet II. 

according to cisco "Ethernet II is the Ethernet frame format used in TCP/IP networks." So what about the 802.3 Frame ? I'm nor really able to find any explanation....

Thank you in advance and regards

Adam 

EDIT: Did a small test in Packet Tracer and Wireshark and the Frame is actually an Ethernet II frame, as an example:

[Peter Paluch · Accepted solution]

Hi Adam,

I apologize for responding late, I had a somewhat busier day.

Hmm, so if the Length Filed (in a 802.3 container plus 802.2 LLC header Frame) is indicating how long the entire "802.2 Header and Data field" is, so I cannot imagine for what this filed could be use  . . . or is it purely to distinguish between a Ethernet II and 802.2 LLC , 802.2 SNAP frame, no less and more ? 

Well, the Length field is exactly what it tells you: It expresses the size of the entire payload portion of the frame, in octets.

Recall that the basic 802.3 container consists of the following fields or parts:

DstMAC, SrcMAC, Length, Message, Checksum

Notice that except the Message parts, all other fields have a fixed length: DstMAC and SrcMAC are 6 bytes long, Length is 2 bytes long, Checksum is 4 bytes long. Only the Message is of a variable size, and the Length field carries the size of the entire Message part. The Message itself may be internally structured into various subfields - it may contain LLC header, SNAP header, other headers, and then the actual message carried by the frame, but to the Length, this does not matter. It describes the size of the entire Message part, no matter how it is structured internally.

You may have a valid question whether the Length field is really required if the Ethernet II can work without it. Obviously, technically, you can do without it just nicely. IEEE must have felt at the time that having a Length field was a natural thing - this feeling has ultimately proven not to be particularly useful. On the other hand, it does not harm, either.

Also, when i look again at your Wireshark capture (on the first packet) and i browse the IEEE 802.3 Ethernet Frame I see the Length value (is it in bits? ) but I'm missing the 802.2 Header and Data Filed.

The value 105 in the Length field is the combined size of the "Logical-Link Control" field (DSAP+SSAP+Control = 3 bytes) and of the Spanning Tree Protocol message (102 bytes). Remember, what you (and Cisco) call "802.2 Header and Data Field" is the 802.2 LLC consisting of DSAP+SSAP+Control (you see it in Wireshark as "Logical-Link Control", and the data which is the Spanning Tree Protocol message itself.

As always, you are welcome to ask further!

Best regards,
Peter

[AdamBudzinski]

Hi Peter,

I need to print out this conversation ! Thank you very much I received more then i would expect, you made it more understandable for me, your help is really appreciated!  

Have a great day and weekend Peter ! Looking forward to hear to hear from you next time :)

Best Regards

Adam 

[martingreen]

Hi,  Does anyone know - if I look at an ethernet II transmission at the wire level it does not seem to contain the 0xfd at the end for end of frame.  I cannot find any reason for this.  Is this because the interpacket gap acts as the end of frame?

Thanks,

Martin

[Peter Paluch]

Hi Adam,

One more thing I forgot to mention.

Many websites out there focus on 802.3 Ethernet - not just on 802.3 Ethernet frame! You have to distinguish between these two.

When Ethernet was first standardized by IEEE after DIX asked them to do so, the original standard was called IEEE 802.3 (and is called this way till today). This standard covers both the physical and the data link layer of the Ethernet as stipulated by IEEE - operation on twisted pair or fiber, CSMA/CD, differences in physical layer for different Ethernet speeds, etc. For different speeds and media types, the physical layer needs to be obviously different. However, the framing is always the same - and in the IEEE 802.3 standard, the framing was defined to be the basic container I've explained earlier:

DstMAC, SrcMAC, Length, Data, FCS

So you perhaps need to keep a certain distance whenever a website or a document talks about "802.3 Ethernet". Do they mean the Ethernet as a technology we all know today, including all frame flavors including LLC, SNAP, and Ethernet II? Or do they specifically focus on the particular frame format that differed from Ethernet II? The term "802.3" is overloaded in this sense.

Whenever you learn about Ethernet in CCNA, you in fact learn about 802.3 Ethernet. But this 802.3 Ethernet validly recognizes different frame formats: the 802.2 LLC format, the SNAP format, and the most widely used Ethernet II format.

Best regards,
Peter