| SOME OF THE MAIN TOPICS IN THIS CHAPTER ARE How Switches Work Switch Hardware Types Switch Troubleshooting and Management Hubs are legacy network devices that enabled creating larger LANs ”extending the distance of the network ”by connecting and centralizing the wiring of multiple physical LAN segments. Bridges were developed to connect separate LAN segments and reduce the collision domain. This chapter explains network switches, which have replaced hubs in all but the oldest networks. In a small home office, a hub might be sufficient to provide LAN connectivity for a few computers. However, in a large, modern network, a more powerful device is needed. It isn't enough to extend the distance of a LAN, which a simple repeater or bridge can do. It isn't enough to centralize wiring, which a hub can do. Note The term collision domain generally refers to the first Ethernet devices, those based on coaxial cable networks, and even those based on hubs. Because early Ethernet networks used a shared network media, each node on the network had to contend for access to the network. The mechanism used to get access to the shared media is called Collision Sense Multiple Access/Collision Detect (CSMA/CD). A computer attempting to transmit data on the shared media first listens to ensure that no one else is already transmitting (collision sense). If the line is free, the computer can begin to transmit data. Yet, because the length of the cables and/or hubs that make up the shared media can be lengthy, it's possible that another node may sense the media to be available at the same time, and start transmitting (multiple access). When this happens, a collision occurs. The collision can be detected because it generates a higher voltage on the wire. The collision domain consists of all those computers (or other devices) that must compete in the same shared media, be it a single cable or many cables interconnected by hubs. Switches have solved this problem in modern Ethernet equipment. | Today's large networks need to provide far more to client and server computers alike, and the device that has become predominant in the LAN for this purpose is the switch. In this chapter, you'll learn that a switch is nothing more than a combination of earlier technologies, with improvements. Without switches, Ethernet networks would have been maxed out a few years ago with 100BASE-T (Fast Ethernet) hub solutions. Note Routers can be used to extend a network. However, the difference between a network router and a LAN switch is significant. Routers are used to direct network frames to the correct network or subnet on which the destination host resides, or to another router that may know of the destination network that lies in the path to the destination network. Although routers can be used within large networked environments to segregate physical network segments, they are generally considered to be WAN devices ”used to connect a LAN to the Internet or a larger private intranet. Switches , in the form most used today, are used to get past the limitations imposed by traditional LAN technologies, including the bus topology, the hub, and the bridge. However, at the end of this chapter, you'll see how switching technology has also moved up the ladder into the WAN market. A major difference between routers and LAN switches is that a router makes decisions based on the network portion of the IP address, whereas switches work at a lower level and make decisions based on the Media Access Control (MAC) address burned into the network card by the manufacturer. Because the MAC address space is random, and the IP address space is hierarchical, LAN switches would need a routing table so large that it would not be possible to store every MAC address for every computer in the world today. Yet routers can use the network portion of an IP address to send a packet on its way because of the very nature of the hierarchical address space (network address/client address) provided by IP. LAN switches can work quickly because they only need to look at the MAC address, which is always located in the same part of the Ethernet frame that carries the IP packet as its payload. |  |
