Switches

Just as having many people in a room trying to speak can result in nobody hearing anything intelligible, using hubs in anything but a small network is not efficient. To improve performance,

LANs are usually divided into multiple smaller LANs interconnected by a Layer 2 LAN switch. The devices connected to a switch again appear as they are all on one LAN, but this time, multiple conversations between devices connected through the switch can happen simultaneously.

NOTE This section discusses Layer 2 LAN switches. The later section "Switching Types" introduces Layer 3 switching.

LAN switches are Layer 2 devices and have some intelligence—they send data to a port only if the data needs to go there. A device connected to a switch port does not receive any of the information addressed to devices on other ports. Therefore, the main advantage of using a switch instead of a hub is that the traffic received by a device is reduced because only frames addressed to a specific device are forwarded to the port on which that device is connected.

Switches read the source and destination MAC addresses in the frames and therefore can keep track of who is where, and who is talking to whom, and send data only where it needs to go. However, if the switch receives a frame whose destination address indicates that it is a broadcast (information meant for everyone) or multicast (information meant for a group), by default it sends the frame out all ports (except for the one on which it was received).

All devices connected to one switch port are in the same collision domain, but devices connected to different ports are in different collision domains. By default, all devices connected to a switch are in the same broadcast domain.

Switches Versus Bridges

You might have also heard of bridges. Switches and bridges are logically equivalent. The main differences are as follows:

■ Switches are significantly faster because they switch in hardware, whereas bridges switch in software.

■ Switches can interconnect LANs of unlike bandwidth. A 10-Mbps Ethernet LAN and a 100-Mbps Ethernet LAN, for example, can be connected using a switch. In contrast, all the ports on a bridge support one type of media.

■ Switches typically have more ports than bridges.

■ Modern switches have additional features not found on bridges; these features are described in later chapters.

Switches do not allow devices on different logical LANs to communicate with each other; this requires a router, as described in the next section.

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