Local-Area Networks and LAN Switching - CCIE Routing and Switching Exam Certification Guide - page 114

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SNAP = 0xAA

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X.25 = 0x7E

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IP = 0x06

In Ethernet networks, the control field is set to 0x03 to indicate an 802.2 unnumbered format
for connectionless service.

IEEE 802.3 SNAP Frame Format
The SNAP field was created to help transition protocols from Ethernet V2 to an IEEE 802.3
compliant frame. It uses the same type values as Ethernet V2, but the EtherType field is
preceded by a 3-byte field for protocol family identification (protocol ID).

The SNAP field includes the Ethernet type information in Ethernet V2. Figure 4-6 shows the
IEEE 802.3 SNAP frame format. The remainder of this section discusses the fields in this
format.

IEEE 802.3 SNAP Frame Format

Figure 4-6

Preamble
7 bytes

SFD
1 byte

DA
6 bytes

SA
6 bytes

Length
2 bytes

LLC
3 bytes

SNAP
5 bytes

Data + pad
38-1492 bytes

FCS

All fields are the same as in the IEEE 802.3 frame format. The SSAP and DSAP of the LLC are
set to 0xAA. The SNAP field is added. The SNAP field contains two fields, including a 3-byte
vendor code field. This 3-byte vendor code is unique to different vendors. The next field in
the SNAP is the 2-byte Ethernet type field in Ethernet V2. The SNAP fields are displayed in
Figure 4-7.

Figure 4-7

IEEE SNAP Fields

Vendor Code
3 bytes

Type
2 bytes

In the years that followed these standards, there has been a mix of frame formats in the network.
IP and DEC protocols use Ethernet V2, Novell's IPX (3.x) uses its 802.3 raw format, and SNA
uses 802.3. As most networks migrate to the IP protocol, Ethernet V2 frames are more common
on the network.

The remainder of this section covers Ethernet physical specifications.