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Cisco CCNA Certification: Defining And Creating Collision 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 "collision domain"
falls into the latter category for many CCNA candidates.
What exactly is "colliding" in the first place,
and why do we care? It's the data that is being sent out onto
an Ethernet segment that we're concerned with here. Ethernet
uses Carrier Sense Multiple Access / Collision Detection (CSMA/CD)
to avoid collisions in the first place. CSMA/CD is a set of
rules dictating when hosts on an Ethernet segment can and
cannot transmit data. Basically, a host that wants to transmit
data will "listen" to the ethernet segment to see
if another host is currently transmitting. If no one else
is transmitting, the host will go forward with its own transmission.
This is an effective way of avoiding a collision, but it
is not foolproof. If two hosts follow this procedure at the
exact same time, their transmissions will collide on the Ethernet
segment and both transmissions will become unusable. The hosts
that sent those two transmissions will then send a jam signal
out onto the segment, indicating to all other hosts that they
should not send data. The two hosts will each start a random
timer, and at the end of that time each host will begin the
listening process again.
Now that we know what a collision is, and what CSMA/CD is,
we need to be able to define a collision domain. A collision
domain is any area where a collision can theoretically take
place, so only one device can transmit at a time in a collision
domain.
In another free CCNA certification tutorial, we saw that
broadcast domains were defined by routers (default) and switches
if VLANs have been defined. Hubs and repeaters did nothing
to define broadcast domains. Well, they don't do anything
here, either. Hubs and repeaters do not define collision domains.
Switches do, however. A Cisco switchport is actually its
own unshared collision domain! Therefore, if we have 20 host
devices connected to separate switchports, we have 20 collision
domains. All 20 devices can transmit simultaneously with no
danger of collisions. Compare this to hubs and repeaters -
if you have five devices connected to a single hub, you still
have one large collision domain, and only one device at a
time can transmit.
Mastering the definition and creation of collision domains
and broadcast domains is an important step toward earning
your CCNA and becoming an effective network administrator.
Best of luck to you in both these worthwhile pursuits!
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage
(http://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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