The Difference Between a Broadcast Domain and a Collision Domain

One of the most confusing things to understand in computer networking is the difference between a broadcast domain and a collision domain. Much of the confusion results because of the operation of a network switch. So to understand where the broadcast domains and collision domain exist within a network, it's easier if you focus on the network switches.

CISCO 16 Port Switch

A switch is a multi-port device that, traditionally, operates at layer 2, the data link layer, of the OSI model. Here packets are sent to a specific switch port based on its destination MAC addresses. Any broadcast traffic on a switch will be forwarded out all ports, so a network switch has no effect on a broadcast domain. However, each port on a switch becomes its own collision domain. So a switch is used to break up a collision domain.

Part of the confusion is because this means that a switch can send broadcasts between collision domains. But that doesn’t mean the switch is joining the collision domains, they're still separate collision domains, all receiving the same broadcasts.

More confusion comes in because, using VLANs (Virtual LANs), a switch can be logically segmented into multiple broadcast domains. So much for a switch not being able to break up a broadcast domain.

Even more confusion comes in because routers operate at layer 3, the network layer, of the OSI model. Routers do not forward broadcasts from one network to another network. But a layer 3 switch is basically a switch that can perform routing in addition to switching, so again we have a switch breaking up a broadcast domain.

And, to add even more confusion, a router can be configured to forward broadcasts from one network to another network. So now, you can't even sat for sure that routers break up broadcast domains.

And, let's not forget to consider the network bridge. To avoid further confusion, just think of a bridge as a single port switch. A bridge operates at layer 2 and breaks up a collision domain, but not a broadcast domain, similar to a switch, but with only one network boundary.

Hubs don't add to the confusion because they don't do anything. They're strictly a device for connection convenience. Each port on a hub is in the same collision and broadcast domain. Repeaters don't add to the confusion because all they do is clean up and amplify the signal. They don't do anything to the collision or broadcast domain.

So if you're asked on an exam, remember the traditional definition is that a switch is a layer 2 device that breaks up a collision domain, but not a broadcast domain. And the traditional definition is that a router is a later 3 device that breaks up a broadcast domain, and of course also a collision domain.

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