Collision and Broadcast Domains
In the previous topic, you gained a better understanding of what a switch is and how it operates. This topic discusses how switches work with each other and with other devices to eliminate collisions and reduce network congestion. The terms collisions and congestion are used here in the same way that you use them in street traffic.
In legacy hub-based Ethernet segments, network devices competed for the shared medium. The network segments that share the same bandwidth between devices are known as collision domains. When two or more devices within the same collision domain try to communicate at the same time, a collision will occur.
If an Ethernet switch port is operating in half-duplex, each segment is in its own collision domain. There are no collision domains when switch ports are operating in full-duplex. However, there could be a collision domain if a switch port is operating in half-duplex.
By default, Ethernet switch ports will autonegotiate full-duplex when the adjacent device can also operate in full-duplex. If the switch port is connected to a device operating in half-duplex, such as a legacy hub, then the switch port will operate in half-duplex. In the case of half-duplex, the switch port will be part of a collision domain.
As shown in the figure, full-duplex is chosen if both devices have the capability along with their highest common bandwidth.
diagram shows that a PC and a switch will autonegotiate matching duplex settings and speeds. In this case full duplex and 100Mb per second