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Cisco CCNA Exam Tutorial: Defining Collision Domains |
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CCNA exam success depends on mastering the fundamentals,
and two important fundamentals are knowing exactly what the
terms "collision domain" and "broadcast domain"
mean. In this free Cisco tutorial, we'll take a look at the
term "collision domain" and how a collision domain
is defined.
A collision domain is an area in which a collision can occur.
Fair enough, but what "collision" are we talking
about here? We're talking about collisions that occur on CSMA/CD
segments, or Carrier Sense Multiple Access with Collision
Detection. If two hosts on an Ethernet segment transmit data
at exactly the same time, the data from the two hosts will
collide on the shared segment. CSMA/CD exists to lessen the
chances of this happening, but collisions can still occur.
To lessen the chances of collisions occurring, we may decide
to create multiple, smaller collision domains.
Let's say we have four hosts on a single Ethernet segment.
The entire segment is a collision domain; any data sent by
one of the hosts can collide with data sent by any of the
other hosts. We have one collision domain containing four
devices.
To create smaller collision domains, we'll need to introduce
some type of networking device into this example. Hubs and
repeaters have their place as far as extending the reach of
a network segment and cutting down on attenuation, but these
OSI Layer One devices do nothing to define collision domains.
We could connect each host into a separate port on a hub (a
hub is basically a multiport repeater) and we'd still have
one single collision domain with four hosts in it.
The most common and most effective way to create multiple
collision domains is to use a switch. If we connect each of
these four hosts to their own separate switch port, we would
now have four separate collision domains, each with one host;
each switch port actually acts as a single collision domain,
making collisions between these four hosts impossible.
Passing the CCNA is all about knowing the details of how
things work, and knowing CSMA/CD theory and how to define
collision domains is one of the many details you've got to
master. In the next part of this CCNA tutorial, we'll take
a look at broadcast domains, and how defining broadcast domains
in the right places can dramatically cut down on unnecessary
traffic on your network.
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage,
home of free CCNA and CCNP tutorials, The Ultimate CCNA Study
Package, and Ultimate CCNP Study Packages.
For a FREE copy of his latest e-books, “How To Pass
The CCNA” and “How To Pass The CCNP”, visit
the website and download your free copies. You can also get
FREE CCNA and CCNP exam questions every day! Pass the CCNA
exam with The Bryant Advantage!
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