34
Chapter 1
Internetworking
doesn't specify in the frame, so it must be IPX. Why? Because when Novell
created the 802.3 frame type (before the IEEE did and called it 802.3 Raw),
Novell was pretty much the only LAN server out there. So, Novell was
assuming that if you're running a LAN, it must be IPX, and so, they didn't
include any Network layer protocol field information in the 802.3 frame.
Flags: 0x80 802.3
Status: 0x00
Packet Length: 64
Timestamp: 12:45:45.192000 06/26/1998
Destination: ff:ff:ff:ff:ff:ff Ethernet Broadcast
Source: 08:00:11:07:57:28
Length: 34
802.2 and SNAP
Since the 802.3 Ethernet frame cannot by itself identify the upper-layer (Net-
work) protocol, it obviously needs some help. The IEEE defined the 802.2
LLC specifications to provide this function and more. Figure 1.13 shows the
IEEE 802.3 with LLC (802.2) and the Subnetwork Access Protocol (SNAP)
frame types.
F I G U R E 1 . 1 3
802.2 and SNAP
Dest SAP
AA
Dest
SAP
Source
SAP
Data
Ctrl
1
1
1 or 2
Variable
Source SAP
AA
Ctrl 03
OUI ID
Type
Data
1
1
1 or 2
3
2
Variable
802.2 (SNAP)
802.2 (SAP)
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