Local-Area Networks and LAN Switching - CCIE Routing and Switching Exam Certification Guide - page 113

Figure 4-4 shows the IEEE 802.3 frame format. Table 4-3 displays the IEEE 802.3 frame fields.

IEEE 802.3 Frame Format

Figure 4-4

Preamble
7 bytes

SFD
1 byte

DA
6 bytes

SA
6 bytes

Length
2 bytes

LLC
3 bytes

Data + pad
43-1497 bytes

FCS
4 bytes

IEEE 802.3 Frame Fields Descriptions Table 4-3

Field Description

Preamble The IEEE defined the preamble as a string of 1s and 0s that is 7 bytes

long.

Start Frame Delimiter (SFD) 1 byte set to 10101011 to indicate the start of the destination address.

Destination Address (DA) 48-bit MAC address of the destination host.

Source Address (SA) 48-bit MAC address of the host that sent the frame (source host).

Length Contains the length of the LLC and the data field in binary. Indicated

values are from 3 to 1500 bytes.

Logical Link Control (LLC) Identifies the ULP.

Data+Pad Contains ULP information.

Frame Check Sequence (FCS) Contains the 32-bit cyclic redundancy check (CRC) for error detection.

The LLC is specified by IEEE 802.2. The LLC provides connectionless (LLC type 1) and
connection-oriented (LLC type 2) services. You use LLC1 with Ethernet networks and LLC2
in IBM SNA environments. The LLC is divided into three fields, as shown in Figure 4-5.

LLC Fields

Figure 4-5

Control
1 or 2 bytes

DSAP
1 byte

SSAP
1 byte

The destination service access point (DSAP) indicates the destination ULP. The source service
access point (SSAP) indicates the source ULP. Examples of SAP values are as follows:
•

NetBIOS = 0xF0

•

Bridge PDU = 0x42

•

SNA = 0x04, 0x05, 0x0C