Difference Between Collision Domain and Broadcast Domain

This tutorial is going to be focusing on two major things collision domains and broadcast domains. You should aware of two different types of domains when designing a LAN otherwise both of these domains can harm the performance of your network. If you are not aware of the difference between these two domains, this tutorial should help you out.

Collision Domain

A collision domain is a section of a network where data packets can collide with one another when being sent on a shared medium or through repeaters, in particular when using early versions of Ethernet the set of LAN devices whose frames could collide with one another. In Collision domain with hubs, bridges, repeaters and wireless access points as only one device can send and receive at a time. If more than one device tries sending or receiving, the information is lost and irrecoverable it will need to be resent. This can slow down network performance along with making it a security threat.

Broadcast Domain

A broadcast domain is a logical division of a computer network with nodes, in which all nodes can reach each other by broadcast at the data link layer. A broadcast domain can be within the same LAN segment or it can be bridged to other LAN segments.

The definition of a broadcast domain is a set of devices that if one device sends a broadcast frame all other devices will receive that frame in the same broadcast domain. So if devices are in the same IP network they will be able to receive a broadcast message. Having a smaller broadcast domain can improve network performance and improve against security attacks. The more PCs and network devices connected to a single broadcast domain, the more broadcast messages you will have. Remember a broadcast message goes to every PC and network device.

Difference between Collision domain and Broadcast domain

Collision Domain

1. Layer 1 of the OSI model

2. A collision domain is a link where collisions can occur between transmitting devices.

3. Collision Domains are defined by Layer 2 devices

4. Only one device in the collision domain may transmit at any one time

5. Collision domain may not be broadcast domain

Broadcast Domain

1. Layer 2 of the OSI model

2. A Broadcast domain is a link where broadcast can occur between devices.

3. Broadcast Domains are defined by Layer 3 devices

4. All devices in the collision domain can transmit at any  time

5. Broadcast domain may be collision domain

 Find the Collision and Broadcast Domains in Cisco Switch

In Cisco Switch, each port is a separate collision domain, So each connection from a single PC to a Layer 2 switch is ONE Collision domain. For example, if 2 PCs are connected with separate cables to a switch, so we have 2 Collision domains. If this switch is connected to another switch or a router, we have one collision domain more  and total 3 collision domains and every Switch having one broadcast domain by default.

Example: Cisco 24 port Catalyst Switch, collision domains = 24 and broadcast domains=1

Cisco 48 port Catalyst Switch, collision domains = 48 and broadcast domains=1

Collision Domain and Broadcast Domains in Hub

In hubs, number of collision domains no way related to how many ports/hosts or devices are connected together, if they are connected with a repeater, hub, switch or bridge, all these devices are in ONE collision domain and it also having only one broadcast domain by default. A Router is used to separate Broadcast-Domains. So, if a router stands between all these devices, we have TWO broadcast domains.

Example: Hub with 4 ports, collision domains = 1 and broadcast domains=1

Hub with 8ports , collision domains = 1 and broadcast domains=1

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  1. Ajay says:

    Collision domain is the group of hosts in which collision can occur, but broadcast domain conists of all the groups of hosts that can proceed the broadcast frame. Broadcast domain may be collision domain but Collision domain may not be broadcast domain.

  2. Manthri says:

    Broadcast Domain is showes nunmber of host is available.

  3. Kapil says:

    Broadcast domain is the logical address scheme in which a user can acess any other networking address without any routing devicebut in Collision domain that is the combination of the single segment and multiple segments through repeter

  4. Jodo says:

    Hello there, You’ve done a great job. I will definitely digg it and for my part suggest to my friends. I’m sure they’ll be benefited from this web site.

  5. Peri says:

    I dont understand the difference between a collision domain and a broadcast domain.
    Does a hub created a broadcast or collision? I heard a hub creates collision domains but a switch creates collision and broadcast domains?

    • admin says:

      A Hub creates a collision domain and a broadcast domain. Here is the theory behind it.
      Since a Hub is a layer 1 device and runs at half-duplex, there is always the chance (more like a guarantee) that there will be collisions. It doesn’t matter how many PC’s are plugged into the hub they will all be affected by the same collision path and hence all part of the same collision domain which was created due to the fact they were plugged into the hub.

      A Hub also doesn’t understand anything about IP addresses, so when the signal is received by a port on the hub, it’s job is to send that signal to every other device plugged into it (on all ports), this means that if a PC sent a broadcast message, all devices would receive it.

      The big thing to understand here, is that a hub does create a single broadcast and collision domain, but does not make more as we will see with the switch.

      A Switch is a layer 2 device which means it has some intelligence in the form of learning Mac addresses. This means that when a PC with mac address ‘A’ wants to send data to a PC with mac address ‘B’ the switch will only send the data to the corresponding port. This means that multiple pc’s could have conversations at the same time. Switches also make use of buffers which allow data to be held back and sent when the line is free and hence prevents any collisions from occurring. Each port on a switch will be its own collision domain (assuming the port has been configured as half-duplex), in a case where each port has been configured as full-duplex no collisions should ever occur. This means that a switch creates collision domains (1 per port, in theory).

      A Switch however does not understand IP addresses (assuming this is a standard switch, and not a layer 3 switch or multilayer switch) and therefore doesn’t understand broadcast messages. This means that when a broadcast is sent from a host attached to the switch it will send the traffic to all ports. Since all machines on the switch will receive the broadcast message, they are said to be in the same broadcast domain.

  6. Sanjiv says:

    Collision domain–frames sent by one ethernet cards0. create collision with frame sent by other ethernet cards

    Broadcast domain frames sent by one ethernet card to all other ethernet card in domain

  7. Venky says:

    Collision is intersection of two or more data at a particular point which results in Loss of data.

  8. Chakri says:

    Broadcast Domain: A set of all devices that recieve broadcast frames originating from any device within the set. Broadcast domains are typically bounded by routers (or, in a switched network, by VLANs) because routers do not forward broadcast frames.

    Collision Domain: In Ethernet, the network area within which frames that have collided are propagated. Repeaters and Hubs prpagate collisions, LAN switches and bridges do not.

  9. Kazi says:

    thanks for difference between collision domain and broadcast domain, these things are very useful for me.

  10. Prem says:

    I understand the difference between a collision domain and a broadcast domain.Does a hub created a broadcast or collision?

    • admin says:

      Hi Prem,

      Hub creates only one broadcast domain and one collision domain but switch can creates multiple collision domains and one broadcast domain. In switch each port is a separate collision domain.

      Thanks

  11. Abdulla says:

    You have covered difference between collision domain and broadcast domain very nicely. Very excellent info and thanks

  12. Rateesh says:

    Collision domain is the group of hosts in which collision
    can occur but broadcast domain consists of all the groups of
    hosts that can proceed the broadcast frame. Broadcast domain
    may be collision domain but Collision domain may not be
    broadcast domain

    A collision domain is an Ethernet term used to describe a
    network,collection of devices in which one particular
    device sends a packet on a network segment.
    A broadcast domain is where a set of all device on that
    same segment hear all broadcasts sent on that segment.

  13. Scott says:

    what is diff between collision domain and broadcast domain, these things how will be useful
    in hub, repeater, bridge, switch

  14. Markus says:

    Hi!

    Preqequisite is that only one vlan is used for simplicity.

    Router to Router segment = 1 cd and 1 bd

    Host to switch = 1cd

    Switch to Switch = 1 cd

    All host within one vlan = 1bD

    Switch to router = 1 cd

    Makes for your scenarios:

    1st: 11cd , 1 bd

    2nd (if on the left the symbol is a hub, because a hub extends a collision domain): 9cd, 3 bd

    Markus.regards!

  15. Markus says:

    Now you forgot the link between the routers :).

    1 bd = left router to switch and host

    1 bd = router to router

    1 bd = the two switches with the devices connected to it

    Regards!

    Markus

  16. Cragsdale32 says:

    Collision Domains are defined by Switches, where as Broadcast Domains are defined by Routers., You want in theory to increase the number of Collision domains in order to minimize the potential for a collision.

    Broadcast Domains define the area in which you can broadcast messages or in which a router will allow broadcast messages to be sent. Typically you want to in theory increase these as well, to minimize possibility for broadcast storms or other possible issue to arise.

    There are ways to allow broadcasts past the Router, but that is beyound teh scope of my training and beyound the scope of the CCNA exam and curriculum that I am in.

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