| When you're studying to pass the CCNA | | | | forward a broadcast out every single |
| exam and earn your certification, you're | | | | port on that switch except the one upon |
| introduced to a great many terms that | | | | which it was received. However, Cisco |
| are either totally new to you or seem | | | | switches allow the creation of Virtual |
| familiar, but you're not quite sure what | | | | Local Area Networks, or VLANs, that are |
| they are. The term "broadcast domain" | | | | logical segments of the network. A |
| falls into the latter category for many | | | | broadcast sent by one host in a VLAN |
| CCNA candidates. | | | | will not be forwarded out every other |
| A broadcast domain is simply the group | | | | port on the switch. That broadcast will |
| of end hosts that will receive a | | | | be forwarded only out ports that are |
| broadcast sent out by a given host. For | | | | members of the same VLAN as the host |
| example, if there are ten host devices | | | | device that sent it. |
| connected to a switch and one of them | | | | The good news is that broadcast traffic |
| sends a broadcast, the other nine | | | | will not be forwarded between VLANs. |
| devices will receive the broadcast. All | | | | The bad news is that no inter-VLAN |
| of those devices are in the same | | | | traffic at all is allowed by default! |
| broadcast domain. | | | | You may actually want this in some |
| Of course, we probably don't want every | | | | cases, but generally you're going to |
| device in a network receiving every | | | | want inter-VLAN traffic. This requires |
| single broadcast sent out by any other | | | | the use of a router or other Layer 3 |
| device in the network! This is why we | | | | device such as a Layer 3 Switch. (Layer |
| need to know what devices can create | | | | 3 Switches are becoming more popular |
| multiple, smaller broadcast domains. | | | | every day. Basically, it's a switch |
| Doing so allows us to limit the | | | | that can also run routing protocols. |
| broadcasts traveling around our network | | | | These switches are not tested on the |
| - and you might be surprised how much | | | | CCNA exam.) |
| traffic on some networks consists of | | | | That router we just talked about also |
| unnecessary broadcasts. | | | | defines broadcast domains. Routers do |
| Using the OSI model, we find devices | | | | not forward broadcasts, so broadcast |
| such as hubs and repeaters at Layer One. | | | | domains are defined by routers with no |
| This is the Physical layer, and devices | | | | additional configuration. |
| at this layer have no effect on | | | | Knowing how broadcasts travel across |
| broadcast domains. | | | | your network, and how they can be |
| At Layer Two, we've got switches and | | | | controlled, is an important part of |
| bridges. By default, a switch has no | | | | being a CCNA and of being a superior |
| effect on broadcast domains; CCNA | | | | network administrator. Best of luck to |
| candidates know that a switch will | | | | you in both of these pursuits! |