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Cisco CCNA Certification: Defining 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 "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.
At Layer Two, we've 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 one upon which it was received.
However, Cisco switches allow the creation of Virtual Local
Area Networks, or VLANs, that are logical segments of the
network. A broadcast sent by one 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.
The good 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 want this in
some cases, but generally you're going to want 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's a switch that can
also run routing protocols. These switches are not tested
on the CCNA exam.)
That router we just 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!
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage
(/www.thebryantadvantage.com), home of free CCNA and CCNP
tutorials, The Ultimate CCNA Study Package, and Ultimate CCNP
Study Packages. Video courses and training, binary and subnetting
help, and corporate training are also available. Pass the
CCNA exam and CCNP exam with Chris Bryant, CCIE #12933!
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Need Cisco Hardware for your Cert? |
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| Call 813.852.6400 now for more information to find the best router or switch to best help you with your certification exam. Having "real" hands-on experience is extremely beneficial not just for testing, but also ensures you are actually familiar with the device you are working on.
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