To pass the CCNA, you need to understand how and why collision domains are important. Chris Bryant, CCIE #12933, will tell you what you need to know.
To pass the CCNA exam, you need to know the basics, and two of the most important basics are knowing what "collision domain" and "broadcast domain" mean. In this free Cisco tutorial, we'll look at what a "collision domain" is and how it's defined.
The area where a collision can happen is called the collision domain. OK, but what kind of "collision" are we talking about here? We're talking about collisions that happen on Carrier Sense Multiple Access with Collision Detection (CSMA/CD) segments. If two hosts on an Ethernet segment send data at the same time, the data from both hosts will mix on the shared segment. This is less likely to happen because of CSMA/CD, but collisions can still happen. We might decide to make several smaller collision domains so that there are less chances of collisions.
Let's say that four hosts are connected to the same Ethernet segment. The whole segment is a collision domain, which means that any data sent by one host can interfere with data sent by any other host. We have four devices in a single collision domain.
We'll need to add some kind of networking device to this example to make the collision domains smaller. Hubs and repeaters are useful for making a network segment reach farther and reducing signal loss, but these OSI Layer One devices do nothing to define collision domains. We could connect each host to a different port on a hub, which is basically a multiport repeater, and we'd still have one single collision domain with all four hosts in it.
A switch is the most common and effective way to make more than one collision domain. If we connected each of these four hosts to its own switch port, we would have four separate collision domains, each with one host. Each switch port actually acts as a single collision domain, making it impossible for these four hosts to collide.
To pass the CCNA, you have to know a lot of details about how things work. One of these details is the CSMA/CD theory and how to define collision domains. In the next part of this CCNA tutorial, we'll talk about broadcast domains and how defining them in the right places can cut down on unnecessary traffic on your network by a lot.
