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Broadcast Domains and Collision Domains

Mar 22, 2013 10:23 AM

Mark Curtis 33 posts since
Jul 10, 2008

Heres another one for you, but a question that seemed to cause me a lot of headache in my CCNA part 1.



Theres a network with one router, two switches hanging off it on two Fast Ethernet interfaces, and then off both switches a bunch of hosts hanging off that. The question was "assuming there is only on native Vlan (Vlan1), how many broadcast domains / collision domains are there?"



Now forgetting the actual numbers here. Does anyone have a straightforward and good explanation of the definition of a broadcast and a collision domain? I know it is a basic question but I think this would prove useful to many people starting out.



Thanks folks

  • Andreas Thrasivoulou 20 posts since
    Jun 26, 2008
    Currently Being Moderated
    1. Jul 14, 2008 2:27 AM (in response to Mark Curtis)
    Re: Broadcast Domains and Collision Domains


    Hi Mark



    if i have understand correcly the diagram you will have 2 broadcast domain and as many connection the switch has for collision domain.



    Generally each vlan is a broadcast domain and each port of the switch is collision domain. Broadcast domain doesn't travel through the routers or the VLAN



    I hope i was helpful






  • Matt Hyland 1 posts since
    Jun 27, 2008
    Currently Being Moderated
    3. Jul 14, 2008 3:19 AM (in response to Mark Curtis)
    Re: Broadcast Domains and Collision Domains


    Based on the diagram provided, then it would indeed be 2 Broadcast domains and 8 Collision Domains. Basically every port on a switch is its own collision domain.



  • Matthieu Malyga 9 posts since
    Jul 2, 2008
    Currently Being Moderated
    4. Jul 15, 2008 3:03 PM (in response to Mark Curtis)
    Re: Broadcast Domains and Collision Domains


    Hi Mark,



    first of all, sorry for my English, I'm from France.



    Collision Domains



    - layer 1 of the OSI model



    - a hub is an entire collision domain since it forwards every bit it receives from one interface on every other interfaces



    - a bridge is a two interfaces device that creates 2 collision domains, since it forwards the traffic it receives from one interface only to the interface where the destination layer 2 device (based on his mac address) is connected to. A bridge is considered as an "intelligent hub" since it reads the destination mac address in order to forward the traffic only to the interface where it is connected



    - a switch is a multi-interface hub, every interface on a switch is a collision domain. A 24 interfaces switch creates 24 collision domains (assuming every interface is connected to something, VLAN don't have any importance here since VLANs are a layer 2 concept, not layer 1 like collision domains)



    Broadcast Domains



    - layer 2 of the OSI model



    - a switch creates an entire broadcast domain (provided that there's only one VLAN) since broadcasts are a layer 2 concept (mac address related)



    - routers don't forward layer 2 broadcasts, hence they separate broadcast domains






    With all this information, you can say that on your diagram, there are 2 broadcast domains (1 router that separates 2 LAN segments composed by one or many switches, with only 1 VLAN per segment).



    There are 8 collision domains, one per pair of devices connected to each other (switch to router, switch to swich, switch to computer etc...) since we are talking about layer 1 concept (physical connection).



    Hope that helps






    Matthieu (from France)



  • Ian Anguin 1 posts since
    Jun 25, 2008
    Currently Being Moderated
    5. Jul 23, 2008 12:09 PM (in response to Mark Curtis)
    Re: Broadcast Domains and Collision Domains


    Each VLAN is a separate Broadcast Domain...(VLAN = Broadcast Domain)



    Each port on a switch is a separate Collision Domain... Hense a 24 port switch has potentially 24 Collision Domains.






    A Hub has a single Collision Domain regardless of the number of ports.



  • Nicolas MICHEL 164 posts since
    Jul 3, 2008
    Currently Being Moderated
    6. Jul 25, 2008 2:26 PM (in response to Ian Anguin)
    Re: Broadcast Domains and Collision Domains

    Yep and a switch creates only one broadcast domain , unless there is more than one VLAN configured

  • Francisco Cardenas 1 posts since
    Aug 4, 2008
    Currently Being Moderated
    7. Aug 4, 2008 9:39 AM (in response to Nicolas MICHEL)
    Re: Broadcast Domains and Collision Domains


    Considering the firs question here and the diagram..... we have two broadcast domains because the router has two "LANs" connected to it. What does the VLAN have to do here? Can the VLAN be considered as a 3rd broadcast domain?



  • Nahum Lokhande 6 posts since
    Jul 30, 2008
    Currently Being Moderated
    8. Aug 8, 2008 7:19 AM (in response to Matthieu Malyga)
    Re: Broadcast Domains and Collision Domains


    Thanks Brother from frans side,



    Praise the Lord.But dear my one Q is still not answered is



    what is it mean exactly the word " colison domain" & "brodcast domain" ?? what happen exatcly with data or frame or packet?






  • Thomas Lee 63 posts since
    Jul 14, 2008
    Currently Being Moderated
    9. Aug 10, 2008 12:48 PM (in response to Mark Curtis)
    Re: Broadcast Domains and Collision Domains


    Broadcast domain is the set systems that will receive a broadcast when sent by a single system. Broadcast domains are always (well usualy!) separated by a route since a router doesn't propogate broadcasts. So in the scenario of a router with two non-connected switches, there are two broacast domains (that seen by switch1 and the other seen by switch 2).



    Collision domain is that set of systems in which a CSMA type collision can occur. WIth switches, each port is it's own collision domain.









  • Chetan 539 posts since
    Jul 26, 2008
    Currently Being Moderated
    10. Aug 12, 2008 10:38 AM (in response to Nahum Lokhande)
    Re: Broadcast Domains and Collision Domains





    A collision domain is a physical network segment where data packets can "collide" with one another for being sent on a shared medium, in particular in the Ethernet networking protocol. This is an Ethernetterm used to describe a network scenario wherein one particular device

    sends a packet on a network segment, forcing every other device on that

    same segment to pay attention to it.



    A group of Ethernet or Fast Ethernet devices in a CSMA/CD LAN that are connected by repeaters and compete for access on the network. This situation is typically found in a hub environment where each host segment connects to a hub that represents only one collision domain and only one broadcast domain.

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

    and the other devices in the domain listen to the network in order to

    avoid data collisions. Collisions decrease network efficiency; if two

    devices transmit simultaneously, a collision occurs, and both devices

    must retransmit at a later time.



    The basic strategy goes like this:


    • A computer listens on the cable to see if another computer is
      transmitting, which is indicated by a voltage change on the cable. If
      busy, the computer waits and listens.

    • When the cable is not busy, a computer attempts to transmit.

    • Another computer may attempt to transmit at the same time, which causes a collision.

    • Both computers that attempted to transmit must back off, wait a random period of time, and then attempt to transmit again.


    Computers on the network detect collisions by looking for abnormally

    changing voltages. Signals from multiple systems overlap and distort

    one another. Overlapping signals will push the voltage above the

    allowable limit. This is detected by attached computers, which reject

    the corrupted frames (called runts).









    A broadcast domain is a logical division of a computer network, in which all nodes can reach each other by broadcast at the data link layer.



    In terms of current popular technologies: Any computer connected to the same Ethernet repeater or switchis a member of the same broadcast domain. Further, any computer

    connected to the same set of inter-connected switches/repeaters is a

    member of the same broadcast domain. Routers and other higher-layer devices form boundaries between broadcast domains.



    I hope above information helps..






  • dante 3 posts since
    Dec 10, 2008
    Currently Being Moderated
    11. Dec 10, 2008 7:56 PM (in response to Mark Curtis)
    Re: Broadcast Domains and Collision Domains

    Hello guys, im a newbie here, yeah i got confused sometimes with Broadcast domains and Collision Domains, before i really dont know whats the difference of this both domains! but now, as reading your reply post, i think im getting to understand the difference of it. Ive a question, what if (2) switches1&2 has 3 host connected to each of it then both switch1&2 is connected to another switch (that is switch3). How many collisions would that be? 1 switch with 3 host, another switch with 3 host then this both switches are connected to another switch (that would make 3 switches in all) ?:|

  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,398 posts since
    Oct 7, 2008
    Currently Being Moderated
    12. Dec 10, 2008 9:40 PM (in response to dante)
    Re: Broadcast Domains and Collision Domains


    There's a collision domain between each host and the respective switches, as well as between the switches themselves. So each individual link is a collision domain.



    The broadcast domain would be defined by your vlan scope.












  • Conwyn 7,907 posts since
    Sep 10, 2008


    Hi Dante



    Maybe history will help.



    Originally all the PC were connected along a piece of co-axial cable so Ethernet could collid. We split these cables with bridges to reduce the collisions but a PC would broadcast to all the PC even on the other side of the bridge.



    Eventually each PC had its own cable to the hub so the PC could only collid with the hub port and eventually we moved to full duplex hubs called switches. You could still broadcast to every PC attached to the switch.



    Collisions still occur with wireless but that is a different story.



    Regards Conwyn



  • dante 3 posts since
    Dec 10, 2008
    Currently Being Moderated
    14. Dec 11, 2008 6:04 PM (in response to Mark Curtis)
    Re: Broadcast Domains and Collision Domains




    In this diagram, there would be 9 collisions right?










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