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