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Switches

The next step in the evolution of Ethernet was the switch. Switches differ from hubs in that switches play an active role in how frames are forwarded. Remember that a hub simply repeats every signal it receives via any of its ports out every other port. A switch, in contrast, keeps track of which devices are on which ports, and forwards frames only to the devices for which they are intended.

Note

What we refer to as a packet in TCP/IP is called a frame when speaking about hubs, bridges, and switches. Technically, they are different things, since a TCP packet is encapsulated with Layer-2 information to form a frame. However, the terms “frames” and “packets” are often thrown around interchangeably (I’m guilty of this myself). To be perfectly correct, always refer to frames when speaking of hubs and switches.

When other companies began developing switches, Cisco had all of its energies concentrated in routers, so it did not have a solution that could compete. Hence, Cisco did the smartest thing it could do at the time—it acquired the best of the new switching companies, like Kalpana, and added their devices to the Cisco lineup. As a result, Cisco switches did not have the same operating system that their routers did. While Cisco routers used the Internetwork Operating System (IOS), the Cisco switches sometimes used menus, or an operating system called CatOS (Cisco calls its switch line Catalyst; thus, the Catalyst Operating System was CatOS).

A quick note about terminology: the ...

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