Collision Domain vs Broadcast Domain

In networking, different network devices are used for various reasons. Hubs, switches, routers and other devices are used to provide specific roles in a network. There are some terms which is very important for these devices and their roles. These terms are Collision Domain and Broadcast Domain. Here, we will learn the details of these terms and we will explain them one by one. We will also compare these two terms and focus Collision Domain vs Broadcast Domain. We will also give different examples with different topologies to understand the concept of these terms. This lesson is simple but important for Cissco CCNA 200-301 Certification.

Before explainning these terms, let ‘s firstly explain domain. What is a domain in networking? A domain is a network that has different network devices connected to it.

What is Collision Domain?

Collision domain is a single physical line that a collision can occur. In other words, it is a network domain that allows only one device to transfer data at a time. If one more device tries to send data at the same time in a collision domain, collision occurs. This means that there will be data loss. Let’s give some examples for collision domain.

For example, simple hubs have one collision domain and one broadcast domain. So, only one node that is connected to this hub can make a data transfer at any time. If another device tries to make data transfer, this data is not reached to the destination.

1 hub, 1 Collision Domain

5 Port Hub, 1 Collision Domain

Another good example for collision domain is switches. By default, each port of a switch is one collision domain. In other words, the number of collision domains for a switch is related with the ports of the switch by default. If a switch has 24 ports, it has 24 collision domains.

3 Switch Ports, 3 Collision Domains

24 Port Switch, 24 Collision Domains

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For routers, this is similar to switches. Each port of a router is one collision domain.

3 Router Ports, 3 Collision Domains

10 Port Router, 10 Collision Domains

What is Broadcast Domain?

Broadcast Domain simply shows a network. In other words, it is the area that a broadcast traffic can sent and received. All the devices that receive this broadcast traffic are in the same broadcast domain. If a device sends broadcast traffic, all the other devices in the same broadcast domain receive this broadcast traffic.

To understand this better, let’s give an example. A hub is one broadcast domain.

1 hub, 1 Broadcast Domain

5 port hub, 1 Broadcast Domain

A Switch is one broadcast domain by default. Because, by default there is no extra VLAN configuration on switches. In such a switch, without any restriction, if one node connected to this switch sends broadcast data, all the other ports receive it. But if we use VLANs in a switch, these VLANs become separate broadcast domains. So, only the VLAN members’ ports receive broadcast traffic.

3 Switch Ports, 1 Broadcast Domain

24 Port Switch, 1 Broadcast Domain

Another example for broadcast domains is router ports. Each router port is one broadcast domain. By default, router do not pass broadcast traffic. So, devices connected to the different port of a router, do not receive any broadcast traffic coming from another port.

3 Router Ports, 3 Broadcast Domains

10 Port Router, 10 Broadcast Domains

VLANs and Collision/Broadcast Domains

VLANs (Virtual Local Area Networks) help you to build new child broadcast domains in one switch or in one broadcast domain. After Configuring VLANs, each VLAN creates a separate Layer 2 broadcast domain and without routing, there is no communication between Virtual LANs.

Remember the previous company example. If we add separate teams to their own VLAN, then different teams cannot send broadcast traffic to the other team. Because, with this configuration, different teams are in different broadcast domains.

There are also Collision Domains in the Virtual LANs again. Each VLAN has Collision Domain as the number of their assigned ports.

Here, VLANs (Virtual Local Area Networks) can be thought like small switches in the main switch.

What is the Difference Between Collision Domain and Broadcast Domain?

A collision domain is a network segment in which Ethernet frame collisions can occur, while a broadcast domain is the part of a network that receives the same Layer 2 broadcast traffic. In modern switched Ethernet networks, each switch port/link forms a separate collision domain, while devices within the same VLAN normally belong to the same broadcast domain. Switches separate collision domains, whereas routers and VLAN boundaries separate broadcast domains.

Collision Domain and Broadcast Domain Examples

Here, let’s give some examples for both broadcast domains and collision domains. With these examples, you will understand these two terms better.

How Many Collision Domains Are There in These Topologies?

Collision Domain Calculation Example 1

Collision Domain Calculation Example 2

How Many Broadcast Domains Are There in These Topology?

Broadcast Domain Calculation Example 1

Broadcast Domain Calculation Example 2

Collision Domain and Broadcast Domain Questions

We have compared these two terms and we have learned Collision Domain vs Broadcast Domain. To explain Collision Domain and Broadcast Domain better, let’s give more examples. You can solve these examples to reinforce your knowledge.

QUESTIONS

ASWERS

Q331 B, Q391 A, Q401 E, Q491 E, Q741 D, Q831 A, Q841 A, Q921 E

Here, we have focused on Collision Domain and Broadcast Domain concepts. We have learned Collision Domain vs Broadcast Domain.

You can check all CCNA Flashcard Questions here.

Frequently Asked Questions About Collision and Broadcast Domains

Does a Switch Break Collision Domains?

Yes. Each switched Ethernet port/link forms a separate collision domain. In modern full-duplex switched Ethernet, collisions normally do not occur.

Does a Switch Break Broadcast Domains?

A Layer 2 switch does not separate broadcast domains within the same VLAN. Broadcast frames are flooded to eligible ports belonging to that VLAN.

Does a Router Break Broadcast Domains?

Yes. Routers separate broadcast domains because Layer 2 Ethernet broadcasts are not normally forwarded between routed interfaces.

How Many Collision Domains Does a Switch Have?

Each active point-to-point switched Ethernet link represents a separate collision domain. For example, four connected devices using four switch ports create four separate collision domains.

How Many Broadcast Domains Does a Switch Have?

A Layer 2 switch has one broadcast domain per VLAN. For example, if a switch has three active VLANs, it has three separate broadcast domains.

Do VLANs Break Broadcast Domains?

Yes. Each VLAN creates a separate Layer 2 broadcast domain. Communication between VLANs requires Layer 3 routing.

Do VLANs Break Collision Domains?

VLANs primarily separate broadcast domains. In a switched Ethernet network, collision domains are already separated by individual switch ports/links.

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gokhankosem

Gokhan Kosem is a Network Engineer, Instructor and the Founder of IPCisco.com with 15+ years of experience in Cisco, Nokia, Huawei, Juniper, Linux, Service Provider Networks, Routing and Switching technologies.

He has worked on the backbone networks of major service providers and network vendors including Nortel, Alcatel-Lucent (Nokia) and has extensive hands-on experience with Cisco, Huawei, Juniper and Nokia networking technologies.

He has trained thousands of networking students worldwide through IPCisco.com, Udemy, books, labs, quizzes, and educational content across multiple social media platforms.

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