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)" shown in Figure 1-4, which shows the various Ethernet frame formats. 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-NetworkAccessProtocol (SNAP) header after the 802.2 header, as shown in the third style of header in Figure 1-4. Later, in 1997, the IEEE added the original DIX V2 framing to the 802.3 standard as well.

Table 1-3 lists the header fields, along with a brief explanation. The more important fields are explained in more detail after the table.

Figure 1-4 Ethernet Framing Options

KEY Ethernet (DIX) and Revised (1997) IEEE 802.3

POINT 8 6 6 ^ 2 Variable 4

KEY Ethernet (DIX) and Revised (1997) IEEE 802.3

POINT 8 6 6 ^ 2 Variable 4

			T
Preamble	Dest.	Source	y	Data	FCS
	Address	Address	p
			e

Original IEEE Ethernet (802.3)

Original IEEE Ethernet (802.3)

					D	S
Preamble	SD	Dest. address	Source address	Length	S A P	S A P	Control	Data	802.2 802.3 802.3 802.2 802.3 IEEE 802.3 with SNAP Header 7 16 6 2 Variable 4 Preamble IEEE 802.3 with SNAP Header 7 16 6 2 Variable 4 Preamble D S SD Dest. address Source address Length S A P S A P Control OUI TYPE Data 802.2 SNAP 802.3 Table 1-3 KEY POINT 802.3 Ethernet Header Fields 802.2 SNAP 802.3 Table 1-3 KEY POINT 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. Related Posts Basic Switch Port Configuration FDDI Frame Format - CCDA Layer 2 QoS CoS on 8021QP Ethernet Frame Ethernet - Mpls Network OSI Layers - Interface Ethernet Ethernet Layer 1 Wiring Speed and Duplex Study Material For Cisco Students Free Hosting for Your Website How to make Money by Investing in Cryptocurrency Create Your First Money Making Website Table 1-3 Ethernet Header Fields (Continued) 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. Continue reading here: Types of Ethernet Addresses Was this article helpful? 0 0