When you’re studying to pass the CCNA exam and earn your certification, you’re introduced to a great many terms that are either totally new to you or seem familiar, but you’re not quite sure what they are. The term “broadcast domain” falls into the latter category for many CCNA candidates.

A broadcast domain is simply the group of end hosts that will receive a broadcast sent out by a given host. For example, if there are ten host devices connected to a switch and one of them sends a broadcast, the other nine devices will receive the broadcast. All of those devices are in the same broadcast domain.

Of course, we probably don’t want every device in a network receiving every single broadcast sent out by any other device in the network! This is why we need to know what devices can create multiple, smaller broadcast domains. Doing so allows us to limit the broadcasts traveling around our network – and you might be surprised how much traffic on some networks consists of unnecessary broadcasts.

Using the OSI model, we find devices such as hubs and repeaters at Layer One. This is the Physical layer, and devices at this layer have no effect on broadcast domains.
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When you’re studying to pass the CCNA exam and earn your certification, you’re introduced to a great many terms that are either totally new to you or seem familiar, but you’re not quite sure what they are. The term “collision domain” falls into the latter category for many CCNA candidates.

What exactly is “colliding” in the first place, and why do we care? It’s the data that is being sent out onto an Ethernet segment that we’re concerned with here. Ethernet uses Carrier Sense Multiple Access / Collision Detection (CSMA/CD) to avoid collisions in the first place. CSMA/CD is a set of rules dictating when hosts on an Ethernet segment can and cannot transmit data. Basically, a host that wants to transmit data will “listen” to the ethernet segment to see if another host is currently transmitting. If no one else is transmitting, the host will go forward with its own transmission.

This is an effective way of avoiding a collision, but it is not foolproof. If two hosts follow this procedure at the exact same time, their transmissions will collide on the Ethernet segment and both transmissions will become unusable. The hosts that sent those two transmissions will then send a jam signal out onto the segment, indicating to all other hosts that they should not send data. The two hosts will each start a random timer, and at the end of that time each host will begin the listening process again.
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To pass the CCNA exam and earn that coveted certification, you’ve got to know Cisco switches inside and out. Among the many important details you’ve got to know are the three methods that Cisco switches use to forward frames, and the differences between the three.

The first switching method is Store-and-Forward. The name is the recipe, because that’s just what the switch does – it stores the entire frame before beginning to forward it. This method allows for the greatest amount of error checking, since the Frame Check Sequence (FCS) can be run before the frame is forwarded. As always, there is a tradeoff, since this error checking process makes this the slowest of the three frame forwarding methods.

The quickest method is Cut-Through, where only the destination MAC address of the frame is examined before the forwarding process begins. This means that the part of the frame is actually being forwarded as it is still being received! The tradeoff here is that the FCS does not run, so there is absolutely no error checking with Cut-Through switching.
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