How a LAN Switch Works

Chapter Description

This chapter explains how frame switching works, and what precisely happens while a frame is switching.

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LAN Switching First-Step

LAN Switching First-Step

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What You Will Learn - Title Page

What You Will Learn

On completing this chapter, you will be able to:

  • Differentiate among unicast, multicast, and broadcast transmission methods

  • Describe store-and-forward, cut-through, and fragment-free switching mechanisms

  • Describe Layer 2 and Layer 3 switching operation

Up to this point, frames going in and out of the LAN switch have been discussed, but not what those frames are doing while in the switch and what the switch is doing with the frames. As you might have surmised by now, this chapter discusses these very points, and a few more. To understand how a switch processes the frames that it receives and forwards, you will first learn about the three types of transmission methods found in a local-area network (LAN): unicast, multicast, and broadcast.

Frames Revisited

Recall from Chapter 1, "Networking Basics," that frames carry data across the network and are made up of three parts: the header, the data itself (payload), and the trailer, as illustrated in the Figure 6-1.

Figure 1Figure 6-1 Complete Frame (Header, Data [Payload], Trailer)

These three frame components—the header, data, and trailer—combine in making up a complete frame. The header identifies the destination data-link address of the frame, the payload is data from upper-layer protocols (such as packets from the network layer), and the trailer signifies the end of the frame.

Recall from Chapter 5, "Ethernet LANs," that the MAC address (Media Access Control address or physical address) is the unique serial number burned into network adapters that differentiates that network card from all others on the network. To be a part of any network, you must have an address so that others can reach you. There are two types of addresses found in a network: the logical network address and the physical data-link address. In LAN bridging and switching environments, you are concerned with the physical address (MAC address), and the MAC address is found in the frame header.

A MAC address is the physical address of the device and is 48 bits (6 bytes) long. It is made up of two parts: the organizational unique identifier (OUI) and the vendor-assigned address, as illustrated in Figure 6-2.

Figure 2Figure 6-2 MAC Address

Recall that the MAC address on a computer might look like this: 00-06-0f-08-b4-12. This MAC address is used for the Fast Ethernet adapter on the computer in question—the OUI is 00-06-0f, and the vendor-assigned number is 08-b4-12.

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