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The table 3-1 is the format of LLC PDU. It comprises four fields, DSAP, SSAP, Control
and Information. The DSAP address field identifies the one or more service access points, in
which the I/G bit indicates it is individual or group address. If all bits of DSAP are 1s, it’s a
global address. The SSAP address field identifies the specific services indicated by C/R bit
(command or response). The DSAP and SSAP pair with certain reserved values indicates
some well-known services listed in the table below.
0xAAAA SNAP
0xE0E0 Novell IPX
0xF0F0 NetBios
0xFEFE IOS network layer PDU
0xFFFF Novell IPX 802.3 RAW packet
0x4242 STP BPDU
0x0606 IP
0x9898 ARP
Table 3-2
LLC type 1 connectionless service, LLC type 2 connection-oriented service
and LLC type 3 acknowledge connectionless service are three types of LLC frame
for all classes of service. In Fig 3-2, it shows the format of Service Access Point
(SAP). Please refer to IEEE802.2 for more details.
Table 3-1 LLC Format