Ccna routing and Switching Complete Study Guide

Collision Domain

In Chapter 1, “Internetworking,” you learned that the Ethernet term collision domain 

refers to a network scenario wherein one device sends a frame out on a physical network 

segment forcing every other device on the same segment to pay attention to it. This is bad 

because if two devices on a single physical segment just happen to transmit simultaneously, 

it will cause a collision and require these devices to retransmit. Think of a collision event as 

a situation where each device’s digital signals totally interfere with one another on the wire. 

Figure 2.1 shows an old, legacy network that’s a single collision domain where only one 

host can transmit at a time.

f i g u r E   2 .1     Legacy collision domain design

One broadcast domain

One collision domain

Hub

Hub

The hosts connected to each hub are in the same collision domain, so if one of them 

transmits, all the others must take the time to listen for and read the digital signal. It is easy 

to see how collisions can be a serious drag on network performance, so I’ll show you how 

to strategically avoid them soon!

Okay—take another look at the network pictured in Figure 2.1. True, it has only one 

collision domain, but worse, it’s also a single broadcast domain—what a mess! Let’s check 

out an example, in Figure 2.2, of a typical network design still used today and see if it’s any 

better.

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Chapter 2 

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  Ethernet Networking and Data Encapsulation 

f i g u r E   2 . 2     A typical network you’d see today

Each connection on a switch creates a separate collision domain.

One broadcast domain by default