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    Broadcast Domains and Collision Domains

    Mark Curtis

      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

        • 1. Re: Broadcast Domains and Collision Domains
          Andreas Thrasivoulou


          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






          • 2. Re: Broadcast Domains and Collision Domains
            Mark Curtis

            Ok thanks, so in the diagram below, there would be 2 broadcast domains (seperated at the router) and then 6 collision domains, or 8 collision domains (i.e. do you count the link from switch to the router as a collision domain too?






            • 3. Re: Broadcast Domains and Collision Domains
              Matt Hyland


              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.



              • 4. Re: Broadcast Domains and Collision Domains
                Matthieu Malyga


                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)



                • 5. 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.



                  • 6. Re: Broadcast Domains and Collision Domains
                    Nicolas MICHEL

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

                    • 7. Re: Broadcast Domains and Collision Domains
                      Francisco Cardenas


                      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?



                      • 8. 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?






                        • 9. 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.









                          • 10. Re: Broadcast Domains and Collision Domains


                            COLLISION DOMAIN



                            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).






                            BROADCAST DOMAIN



                            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..






                            • 11. 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) ?:|

                              • 12. Re: Broadcast Domains and Collision Domains
                                Scott Morris - CCDE/4xCCIE/2xJNCIE


                                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.












                                • 13. Re: Broadcast Domains and Collision Domains


                                  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



                                  • 14. Re: Broadcast Domains and Collision Domains




                                    In this diagram, there would be 9 collisions right?









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