Collision and Broadcast Domains, what are they and how do I accurately count them?

Hey guys,
I'm studying for my CCNA and I still have come confusion about Collision and Broadcast domains. Does Collision refer to sending out packets, and Broadcast is forwarding those packets?
Also, what's the different for routers, switches and hubs? I know routers have different collision and broadcast domains, switches have different collision and same broadcast, and hubs have one collision and broadcast?
When I look at these topologies I am very confused of how many Broadcasts and how many Collision domains there are here and why. Please help.


  • JoeMJoeM ✭✭✭

    Hello Prof,
    That is a lot of questions for a single post. Good questions, but should probably be whittled down a little. Are you going through Keith Bogart's videos on INE? I like the way he breaks things down and explains things.


    Here is a summary excerpt from Wendell Odom's book:

    • A collision domain is a set of NICs for which a frame sent by one NIC could result in a collision with a frame sent by any other NIC in the same collision domain.
    • A broadcast domain is a set of NICs for which a broadcast frame sent by one NIC is received by all other NICs in the same broadcast domain.

    My brief summary of this:
    Collision Domains -- is where frames sent on a wire actually collide with traffic from other devices (frames are wrecked in a collision). This would happen to traffic connected via a hub because it is half-duplex and traffic must compete to put traffic on the wire.
    Bridges and Switches helped with this by isolating collisions to a single connection and more-or-less solves the collision issue with full-duplex (in and out traffic having their own wires).

    Broadcasts Domains - separation of end-devices into groups that can communicate with each other with an L2 broadcast address FFFF.FFFF.FFFF.
    Vlans helped resolve this, by creating smaller isolated broadcast domains. The key here is that an L3 gateway is needed to communicate between different isolated "Broadcast Domain". NOTE that I say L3 (not router), because a Layer-3 capable switch can also give this "router" capability. It facilitates inter-vlan communication by allowing vlans to communicate with each other without leaving the switch -- meaning without using a "router-on-a-stick" method.

    Keith Barker has a short fun video explaining the difference between types of devices:
    Switches, and Routers, and Hubs! Oh my!

    Here are some other youtube videos on the subject of
    Collision vs Broadcast Domains



    And here is my answer for the above two diagrams. I am treating all of the devices in the diagrams as though they are in a single vlan (no router-on-a-stick).

    Image 1:

    **Image 2: ** what do you think? Give it a try.


    Hope this helps!

  • Hey Joe,
    Thanks for the reply. I'm still a bit confused after watching a lot of videos. But, I think you got that one a little wrong. The book said 9 collision domains, and 3 broadcast domains.
    I appreciate the response and any advice helps. For the bottom image I have 12 collision domains and 1 broadcast domain. But, what would it be if the router wasn't there? How many broadcast domains would it be?

  • JoeMJoeM ✭✭✭

    Okay. I am missing 2 collision domains.
    I suppose those would be the inter-switch connections on the left side. I was only counting the end-devices.

    For the second image, you ask what happens if we removed the router. Broadcast domain would still be the same. but we would have one collision domain less.

  • I understand. Thanks a lot for clearing that up Joe. It really helped. I appreciate the support :)

Sign In or Register to comment.