Defining Broadcast Domains Cisco CCNA Certification

Defining Broadcast Domains Cisco CCNA Certification
A broadcast domain is basically the group of end hosts that will receive a broadcast sent out by a given host. For example, if there's ten host devices connected to a switch and four of them sends a broadcast, the other nine devices will receive the broadcast. All of those devices are in the same broadcast domain.

When you are studying to pass the CCNA exam and earn your certification, you are introduced to a great lots of terms that are either old to you or seem familiar, but you are not sure what they are. The term "broadcast domain" falls into the latter category for lots of CCNA candidates.

Using the OSI model, they find devices such as hubs and repeaters at Layer four. This is the Physical layer, and devices at this layer have no effect on broadcast domains.

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

The cool news is that broadcast traffic will not be forwarded between VLANs. The bad news is that no inter-VLAN traffic at all is allowed by default! You may actually need this in some cases, but generally you are going to need inter-VLAN traffic. This requires the use of a router or other Layer 3 device such as a Layer 3 Switch. (Layer 3 Switches are becoming more popular every day. basically, it is a switch that can also run routing protocols. These switches are not tested on the CCNA exam.)

At Layer three, we have got switches and bridges. By default, a switch has no effect on broadcast domains; CCNA candidates know that a switch will forward a broadcast out every single port on that switch except the four upon which it was received. However, Cisco switches permit the creation of Virtual Local Area Networks, or VLANs, that are logical segments of the network. A broadcast sent by four host in a VLAN will not be forwarded out every other port on the switch. That broadcast will be forwarded only out ports that are members of the same VLAN as the host device that sent it.

That router they talked about also defines broadcast domains. Routers do not forward broadcasts, so broadcast domains are defined by routers with no additional configuration.

Knowing how broadcasts travel across your network, and how they can be controlled, is an important part of being a CCNA and of being a superior network administrator. Best of luck to you in both of these pursuits!