Ethernet Layer 2: Framing and Addressing
In this book, as in many Cisco courses and documents, the word frame refers to the bits and bytes
that include the Layer 2 header and trailer, along with the data encapsulated by that header and
trailer. The term packet is most often used to describe the Layer 3 header and data, without a Layer
2 header or trailer. Ethernet’s Layer 2 specifications relate to the creation, forwarding, reception,
and interpretation of Ethernet frames.
The original Ethernet specifications were owned by the combination of Digital Equipment Corp.,
Intel, and Xerox—hence the name “Ethernet (DIX).” Later, in the early 1980s, the IEEE
standardized Ethernet, defining parts (Layer 1 and some of Layer 2) in the 802.3 Media Access
Control (MAC) standard, and other parts of Layer 2 in the 802.2 Logical Link Control (LLC)
standard. Later, the IEEE realized that the
1-byte DSAP field in the 802.2 LLC header was too small. As a result, the IEEE introduced a new
frame format with a Sub-Network Access Protocol (SNAP) header after the 802.2 header, as shown
in the third style of header in Figure 1-4. Finally, in 1997, the IEEE added the original DIX V2
framing to the 802.3 standard as well as shown in the top frame in Figure 1-40.
Table 1-3 lists the header fields, along with a brief explanation. The more important fields are
explained in more detail after the table.
Ethernet Layer 2: Framing and Addressing 15
Figure 1-4 Ethernet Framing Options
Table 1-3 Ethernet Header Fields
Field Description
Preamble (DIX) Provides synchronization and signal transitions to allow proper clocking
of the transmitted signal. Consists of 62 alternating 1s and 0s, and ends
with a pair of 1s.
Preamble and Start of
Frame Delimiter (802.3)
Same purpose and binary value as DIX preamble; 802.3 simply renames
the 8-byte DIX preamble as a 7-byte preamble and a 1-byte Start of
Frame Delimiter (SFD).
Type (or Protocol Type)
(DIX)
2-byte field that identifies the type of protocol or protocol header that
follows the header. Allows the receiver of the frame to know how to
process a received frame.
Length (802.3) Describes the length, in bytes, of the data following the Length field, up
to the Ethernet trailer. Allows an Ethernet receiver to predict the end of
the received frame.
Destination Service Access
Point (802.2)
DSAP; 1-byte protocol type field. The size limitations, along with other
uses of the low-order bits, required the later addition of SNAP headers.
Source Service Access
Point (802.2)
SSAP; 1-byte protocol type field that describes the upper-layer protocol
that created the frame.
continues
Dest.
Address
Preamble
Source
Address
Type/
Length
Data FCS
Ethernet (DIX) and Revised (1997) IEEE 802.3
8 6 6 2 Variable 4
Original IEEE Ethernet (802.3)
IEEE 802.3 with SNAP Header
7 1 6 6 2 1 1 1-2 Variable 4
Dest.
address
Preamble
DSAP
SSAP
Source
address
SD Length Control Data FCS
802.3 802.2 802.3
7 1 6 6 2 1 1 1-2 3 2 Variable 4
Dest.
address
Preamble
DSAP
SSAP
Source
address
SD Length Control OUI TYPE Data FCS
802.3 802.2 SNAP 802.3
Field Description
Control (802.2) 1- or 2-byte field that provides mechanisms for both connectionless and
connection-oriented operation. Generally used only for connectionless
operation by modern protocols, with a 1-byte value of 0x03.
Organizationally Unique
Identifier (SNAP)
OUI; 3-byte field, generally unused today, providing a place for the
sender of the frame to code the OUI representing the manufacturer of the
Ethernet NIC.
Type (SNAP) 2-byte Type field, using same values as the DIX Type field, overcoming
deficiencies with size and use of the DSAP field.