Ethernet Version 2 goes by many names, as you know: Ethernet_II Ethernet V2 arpa (Cisco) ETHERNET_II (Novell)
Ethernet Version 2 has: dest, src, type. All other types start with an 802.3 header. This is where the confusion comes in. Some descriptions and names for the formats forget this basic fact. If it's not Ethernet V2, it starts with an 802.3 header: dest, src, length See a problem with that? There's no way to identify the next layer up! Novell didn't care about this problem. They assumed that all packets would come into the NetWare Operating System and they could figure out what to do with it from there by looking at the network layer. If the next two bytes after length are the first two bytes of IPX (0xFFFF usually), then it's for IPX. Otherwise, use some other indication at the network layer to determine the type of packet. So Novell raw has just an 802.3 header: dest, src, length Novell raw is also known as: novell-ether (Cisco) ETHERNET_802.3 (Novell) Nobody but Novell would use such a frame format though! ;-) In the rest of the world, the data-link layer has a way of identifying the next layer. In the rest of the world, if you use 802.3 (with a length where the old V2 type used to be), you better also use an 802.2 LLC header. Then the frames look like: dest, src, length, DSAP, SSAP, Control They start with 802.3 and are followed by the 802.2 fields, including Destination Service Access Point (DSAP) and Source Service Access Point (SSAP). Those service access points identify the next layer up. 802.3 with 802.2 is often just called 802.3 since IEEE 802.3 requires the use of 802.2 with 802.3. It is also known as: ETHERNET_802.2 (Novell) sap (Cisco) The IEEE ran out of DSAPs and SSAPs, so they had to tell some protocols, like AppleTalk and most Cisco management protocols, like CDP, to use yet another header, the SNAP header. So then the frames look like: dest, src, length, DSAP, SSAP, Control, Vendor, Type The DSAP and SSAP are 0xAA for SNAP. The SNAP Type field identifies the next layer up. A couple more comments below... [EMAIL PROTECTED] wrote: > > Peeps, > > I am in MELT DOWN currently :( and Arsenal have not even > played the > semi-final > at old trafford yet. (thats is tomorrow!) - Come on the > Gunners! > > Could you help me with this one question? > > > > The four frame formats are as follows: > > Ethernet version 2 > Novell 802.3 Raw > IEEE 802.3 > IEEE 802.3 Subnetwork Access Protocol (SNAP) That's one way of naming them. ;-) Note that IEEE 802.3 also uses 802.2, (unless it's Novell raw). And with SNAP, there's also an 802.2 header between 802.3 and SNAP. > > or > The four frame formats are as follows: > > ETHERNET 802.3 > ETHERNET 802.2 > ETHERNET SNAP > ETHERNET II > That's Novell's naming. > > > I can relate Ethernet v2 to Ethernet II > I can relate IEEE 802.3 to IEEE 802.3 When Novell says 802.3 they mean their raw format where the data link header is immediately followed by the network layer header. > I can relate IEEE 802.3 Subnetwork Access Protocol (SNAP) to > ETHERNET SNAP > > Should I just relate Novell 802.3 Raw to ETHERNET 802.2 and > forget about the > whole thing? > Is this even correct to relate these two types together? No, when Novell says ETHERNET_802.2 they mean 802.3 followed by 802.2, the method used by the entire world except Novell, that is, when IEEE format is used at all. (IP uses Ethernet V.2) Sorry, it's so confusing! :-) > > Summary of my understanding is:- > EII has TYPE field > 802.3 has LENGTH field > 802.2 has 1 byte DSAP and SSAP fields > SNAP improves on 802.2 by having 3 byte ORG CODE field and 2 > byte TYPE field That's right. But SNAP doesn't "improve" on anything. It's just necessary because IEEE ran out of SAPs. This is covered in much detail in Troubleshooting Campus Networks. It's covered a little bit in the errata since the annoying graphic artist changed one field in my figure. And I have a lab exercise that is very helpful for understanding this. Even if you don't actually do the lab, the explanation is helpful. These resources are available here: http://www.troubleshootingnetworks.com/ Priscilla > > > Many thx > > > ------------------------------------------------------------------------ > For more information about Barclays Capital, please > visit our web site at http://www.barcap.com. > > > Internet communications are not secure and therefore the > Barclays > Group does not accept legal responsibility for the contents of > this > message. Although the Barclays Group operates anti-virus > programmes, > it does not accept responsibility for any damage whatsoever > that is > caused by viruses being passed. Any views or opinions > presented are > solely those of the author and do not necessarily represent > those of the > Barclays Group. 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