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Chapter 1

Internetworking

802.2 Logical Link Control (LLC) Header

Dest. SAP: 0xAA SNAP

Source SAP: 0xAA SNAP

Command: 0x03 Unnumbered Information

Protocol: 0x080007809B AppleTalk

You can identify a SNAP frame because the DSAP and SSAP fields are

always AA, and the Command field is always 3. This frame type was created
because not all protocols worked well with the 802.3 Ethernet frame, which
didn't have an Ether-Type field. To allow the proprietary protocols created
by application developers to be used in the LLC frame, the IEEE defined the
SNAP format. Up until last year or so, the SNAP frame was on its way out
of the corporate market. However, the new 802.11 wireless LAN specifica-
tion uses an Ethernet SNAP field to identify the Network layer protocol.
Cisco also still uses a SNAP frame with their proprietary protocol Cisco Dis-
covery Protocol (CDP)--something I'm going to talk about in Chapter 7.

Ethernet at the Physical Layer

Ethernet was first implemented by a group called DIX (Digital, Intel, and
Xerox). They created and implemented the first Ethernet LAN specification,
which the IEEE used to create the IEEE 802.3 Committee. This was a
10Mbps network that ran on coax, twisted-pair, and fiber physical media.

The IEEE extended the 802.3 Committee to two new committees known

as 802.3U (FastEthernet) and 802.3Z (Gigabit Ethernet). These are both
specified on twisted-pair and fiber physical media.

Figure 1.14 shows the IEEE 802.3 and original Ethernet Physical layer

specifications.

F I G U R E 1 . 1 4

Ethernet Physical layer specifications

When designing your LAN, it's really important to understand the differ-

ent types of Ethernet media available to you. Sure, it would certainly be great
to run Gigabit Ethernet to each desktop and 10Gbps between switches, and

Data Link

(MAC layer)

Physical

Ethernet

802.3

10Base2

10Base5

10BaseT

10BaseF

100BaseTX

100BaseFX

100BaseT4

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