802.2 llc / snap

telecomops101

Can someone explain the difference between the 802.2 LLC and SNAP headers. I believe they are both used to identify the network layer protocol being transmitted, but when is one or the other used, or can they be used at the same time. Also, what does DSAP and SSAP mean pertaining to the 802.2 LLC header. Thanks in advace.

Fish!

The whole thing is a terribly complex way to specify the 'next protocol' which is essentially the protocol contained in the ethernet frame.

In most networks nowadays the type/length field in the ethernet header directly specifies the next protocol, such as 0x800 (IP) or 0x0806 (arp).

If the type/length field specifies the length instead of the type, then an 802.2 LLC header follows. 802.2 LLC can do many interesting things (please don't take me back to my SNA days! That Cisco 7500 with the CIP card has long since been de-installed and I would like to forget, not remember!) but basically it is the job of the 802.2 LLC header to then specify the next protocol.

But wait, the 802.2 LLC header only specifies IEEE protocols. What if you want to run something non-IEEE? Then the 802.2 LLC header specifies the next protocol is SNAP, and a SNAP header follows the 802.2 LLC header.

The SNAP header then FINALLY specifies the 'next protocol'. SNAP is basically an augmentation of 802.2 LLC to allow it to specify many many more 'next protocols' than it is capable of doing otherwise.

Like I said, 802.2 LLC can do many different things, including connection-oriented service with acknowledgement at layer 2 (yep! Go look THAT up!). Also, when talking 802.2 LLC, the OSI model data link layer is essentially divided into two sub-layers (LLC on top and MAC below) which you NEVER see in any training or books nowadays, but for the most part it boils down to a complex way to specify next protocol.