6 Replies Latest reply: Jan 23, 2011 1:59 PM by Brian RSS

    collision domain


      Do vlan increase or decrease collision domains?



      Do vlan decrease number of broadcast domains?



      Do bridges decrease broadcast domain?

        • 1. Re: collision domain

          Vlans will increase the number of collision domains and decrease the size of each one.  Remember "collision domains" are layer 2 so each switchport is it's own collision domain.


          Vlans will also increase the number of broadcast domains and decrease the size of each one.  A router would serve the same purpose, routers break up broadcast domains.


          Bridges will do nothing to broadcast domains, you would need a router to break up the broadcasts, or use a switch and implement Vlans.

          • 2. Re: collision domain

            Well, yes and no. 


            VLANs have very little to do with collision domains on current hardware.  With Cisco switches, especially newer ones, each port is a collision domain. To take it one step fruther, in full duplex, that isn't even the case.  In full duplex since both ends of the connection can tx and rx at the same time, collisions are a thing of the past.


            Broadcast domains, however, are still a concern.  Broadcasts are typically not forwarded by routers - so when you hit that first L3 interface, the broadcast stops.  This reduces the scope of a broadcast to the VLAN it originated in. 


            Bridges are older technology, routers took their place.  However, yes, if bridged at the L2 layer, they would increase a given VLANs broadcast scope, assuming you are spanning the VLAN across the bridge. 



            Jackie485, honestly it sounds like you may be reviewing some older materials.  What are you reading that prompted these questions?  I remember seeing this on the older CCNA test I took in 2005, and that was even on it's way out the door.

            • 3. Re: collision domain

              collision domains were or are a term that was used in half duplex non switched environments.  Basically, if two hosts were trying to talk at the same time, a collision would occur.  With full duplex switched ports, this issue is all but gone, although it is still very prevelant in the wireless world.


              If you have hubs, then collisions are an issue.  If you have switches, then each port is its own collision domain.  Even though the collision domain is "old" I still think its good to know and understand it.


              Think of your broadcast domain as when a broadcast happens, who all would hear that broadcast.  When you create a vlan, you are essentially creating a new broadcast domain, so that if a broadcast in performed in vlan 1, any hosts in vlan 2 won't hear it.


              These are the very foundational topics of Layer 2 networking and switching and are still good to know.

              • 4. Re: collision domain
                Paul Stewart  -  CCIE Security

                Do vlan increase or decrease collision domains?


                No effect on the number of collision domains.



                Do vlan decrease number of broadcast domains?


                Each vlan is a broadcast domain.  So I'd say vlan's actually increase the number of broadcast domains linearly based on the number of vlans added.



                Do bridges decrease broadcast domain?


                Bridges would split a collision domain, not a broadcast domain.  So bridges have no effect over the number of broadcast domains, but would increase the number of collision domains.


                I think these are actually current CCNA topics.  I agree that we really don't have collisions in full duplex but Cisco calls each switchport a Collision domain.  This is an Ethernet concept.

                • 5. Re: collision domain
                  Scott Morris - CCDE/4xCCIE/2xJNCIE

                  I'd agree with Paul...


                  As for whether this topic is valid for CCNA or not, the blueprint isn't very specific, but I'd think it would still fit.   And if you don't know, you're destined to have it asked! 


                  Describe how a network works

                  • Describe the purpose and functions of various network devices

                  • Select the components required to meet a network specification

                  • Use the OSI and TCP/IP models and their associated protocols to explain how data flows in a network

                  • Describe common networked applications including web applications

                  • Describe the purpose and basic operation of the protocols in the OSI and TCP models

                  • Describe the impact of applications (Voice Over IP and Video Over IP) on a network

                  • Interpret network diagrams

                  • Determine the path between two hosts across a network

                  • Describe the components required for network and Internet communications

                  • Identify and correct common network problems at layers 1, 2, 3 and 7 using a layered model approach

                  • Differentiate between LAN/WAN operation and features


                  Check out https://learningnetwork.cisco.com/docs/DOC-4976



                  • 6. Re: collision domain

                    Yes, these are still relevent CCENT and CCNA topics and are still covered in the current ICND1 and ICND2 books.


                    I think the basic concept Cisco is still trying to stress is


                    1. Hubs = single collision domain and single broadcast domain

                    2. Switches = multiple collision domains and single braodcast domain

                    3. Routers = multiple collision domains and broadcast domains


                    When you introduce VLANs on the switches, you now have multiple collision domains and multiple broadcast domains.