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Broadcasts, Hubs, Routers, And Switches


In a previous Cisco CCENT certification exam tutorial, we talked about broadcasts and the potential of a broadcast storm. (If you missed that one, visit my website's Tutorials section.) In today's tutorial, we'll discuss several different common network devices and how they help to limit broadcast propagation - or in some cases, how they do not help!

In the "do not help" department, we'll find hubs and repeaters. These two devices operate at Layer 1 of the OSI model (the Physical layer), and their sole purpose is to strengthen the electrical signals sent over the cable. They don't have anything to do with switching or routing, and they do not help to limit broadcasts. (A hub is basically just a repeater with more ports.)

On the other end of the spectrum, we have routers. Routers operate at Layer 3 of the OSI model (the Network layer), and by default routers do not forward broadcasts. They can be configured to "translate" certain broadcast types into unicasts, but you'll learn more about that in your CCNA studies.

Since routers do not forward broadcasts, there's a misconception that routers have nothing to do with broadcasts. Routers can indeed generate broadcasts, and they can accept broadcasts - but they will not forward broadcasts. That's an important distinction.

Between these two extremes, we find switches. Switches operate at Layer 2 of the OSI model (the Data Link layer), and the default behavior of a switch is to accept a broadcast and forward it out every other single port on that switch except the port that first received the broadcast.

If that sounds like a lot of broadcast forwarding, it is! If we have an 80-port switch and one port receives a broadcast, by default a copy of that broadcast is going to be forwarded out the other 79 ports. Most likely, not all of those hosts connected to those switchports need to see that broadcast, and sending unnecessary broadcast results in an unnecessary use of network resources, particularly bandwidth.

Luckily for us, there is a way to configure a Cisco switch to limit which ports receive that broadcast, and we'll take a look at that method in the next installment of my Cisco CCENT certification exam tutorial series!

About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage , home of free CCNA and CCNP tutorials! Pass the CCNA exam with Chris Bryant!





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