Traffic forwarding is based on address learning. In ChapterВ 4, we discuss special purpose switches and their uses.ДЄn Ethernet switch controls the transmission of frames between switch ports connected to Ethernet cables using the traffic forwarding rules described in the IEEE 802.1D bridging standard. The wide range of switch models and capabilities can be confusing. The result has been a large and competitive market in Ethernet switches, increasing the number of choices you have as a customer. Instead, vendors compete with one another to provide switches at multiple price points and with multiple levels of performance and capabilities. While the 802.1D standard provides the specifications for bridging local area network frames between ports of a switch, and for a few other aspects of basic bridge operation, the standard is also careful to avoid specifying issues like bridge or switch performance or how switches should be built. For our purposes, we will follow the practices of Ethernet vendors who use the word вЂњswitch,вЂќ or more specifically, вЂњEthernet switch,вЂќ to describe devices that bridge Ethernet frames.

The point is that as far as network experts are concerned, bridging and routing are different kinds of packet switching with different capabilities. However, note that вЂњswitchвЂќ is a generic term for network devices that may function as bridges, or routers, or even both, depending on their feature sets and configuration. We will use the words вЂњbridgeвЂќ and вЂњswitchвЂќ interchangeably to describe Ethernet bridges. As a result, modern Ethernet networks may consist of hundreds of switch connections in a building, and thousands of switch connections across a campus network. Modern buildings have multiple wireless access points (APs) to provide 802.11 Wi-Fi services for things like smartphones and tablets, and each of the APs is also connected to a cabled Ethernet system. Every VoIP telephone and every printer is a computer, and even building management systems and access controls (door locks) are networked. Over the years, computers have become ubiquitous, and many people use multiple devices at their jobs, including their laptops, smartphones, and tablets.

Things have changed quite a lot since Ethernet bridges were first developed in the early 1980s. Later, when twisted-pair Ethernet was developed and switches with many ports became widely available, they were often used as the central connection point, or hub, of Ethernet cabling systems, resulting in the name вЂњswitching hub.вЂќ Today, in the marketplace, these devices are simply called switches. At that time, Ethernet only supported connections to coaxial cables. The first Ethernet bridges were two-port devices that could link two of the original Ethernet systemвЂ™s coaxial cable segments together. ThatвЂ™s the result of lots of hard work on the part of the standards engineers to define a set of standards that vendors could agree upon and implement in their switch designs. The standardization of bridging operations in switches makes it possible to buy switches from different vendors that will work together when combined in a network design. Ethernet bridging was initially defined in the 802.1D IEEE Standard for Local and Metropolitan Area Networks: Media Access Control (MAC) Bridges. To do this, they copy Ethernet frames from one switch port to another, based on the Media Access Control (MAC) addresses in the Ethernet frames. By moving Ethernet frames between the switch ports, a switch links the traffic carried by the individual network connections into a larger Ethernet network.Д®thernet switches perform their linking function by bridging Ethernet frames between Ethernet segments. Ethernet switches link Ethernet devices together by relaying Ethernet frames between the devices connected to the switches.

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