LLC (IEEE 802.2) and SAPs.

[moacir.ferreira]

The 802.2 (LLC) introduced the source and destination Service Access Points (SSAP and DSAP). However, I never saw a single frame where the source SAP is different from the destination SAP.

Question: What is the use of the source/destination SAP if both are always the same?

[tvanderwerken]

Sure the SAP's are the same if there is a layer 3 protocol, how else would you get through the LLC sub-layer? However, SNA uses's SAP's 04,08,0C,10 to communicate between hosts. Most of the time it will be SSAP 04 and DSAP 04, but you will see a PU with a SSAP of 08 or 0C or 10, sending to a Host on DSAP 04.

HTH

Todd