Switch/Router Architectures

Appendix AEthernet Frame

A.1 Introduction

The IEEE standards for Local Area Networks (LANs) describe specifications for the physical layer, medium access control (MAC) sublayer, and logical link control (LLC) sublayer. In OSI terminology, the MAC and LLC sublayers are considered to be sublayers of the OSI Data Link layer. Both the MAC and LLC sublayers contain fields for addressing. The term Ethernet refers to the family of LAN specifications covered by the IEEE 802.3 standard.

The processing that take place in Layer 2 (Ethernet) switches, switch/routers (multilayer switches), and routers involve processing all or parts of the Ethernet frame and IP header fields. To understand how switches, routers, and switch/routers operate, it is essential to understand the format of the Ethernet frame and IP header.

A.2 Ethernet Frame Format

The IEEE 802.3 standard defines the basic Ethernet frame format that is required for all Ethernet device implementations, in addition to other optional fields that are used to extend Ethernet's basic capabilities. The basic Ethernet frame contains the Preamble plus the six fields shown in Figure A.1.

• Preamble: An Ethernet frame is preceded by a Preamble, which consists of 7 bytes ¹ or octets (56 bits). The Preamble is a pattern of alternating ones and zeros preceding an Ethernet frame that indicates to a receiving device that an Ethernet frame is arriving. It also provides a signal to the receiving device to enable the frame-reception mechanism of ...