What is the difference between a collision domain and broadcast domain?

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what is the difference between collision domain & broadcast domain?

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As stated above, a collision domain is a link where collisions can occur between transmitting devices. For example a hub with several people attached to it or a bus type network connection. Anytime you have more than two devices sharing a common medium (cable, air, etc) there can be collisions. Every port on a switch is a collision domain so an individual pc connected to a switch port has no one to collide with. Broadcasts however, when processed by a switch are flooded out of every port, thus switches are defined as a single broadcast domain. Everyone connected to a switch receives every broadcast generated by any other device connected to that switch. Consider the ramifications if you have 20 or 30 switches nterconnected with one another. A pc on one switch on one side of the network generates a broadcast and that broadcast, even though it may destined to a device right next door, gets flooded to every port on every interconnected switch. Not good. For that reason we put individual ports into VLANS which basically keep traffic seperated by a configured number associated with any specific port. Traffic generated in VLAN1 for example can only be forwarded into VLAN1. There is no interVLAN connectivity unless you put a router in the network and then use it to route between the VLANS. This is whey the devices in one VLAN typically have an IP address in one IP network and devices in other VLANS have addresses associated with a different subnet. VLANS are configured as a layer 3 subnet. The router acts as a border that broadcasts (by default) do not cross. When traffic is generated in one VLAN and needs to go into another VLAN the router simply sees it as subnet to subnet traffic and routes it as normal. If you have further questions please feel free to contact me. Brent Mossberger, bmossberger@gmail.com

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  • CRagsdale32
    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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  • 8870252743
    what is diff between collision domain and broadcast domain, these things how will be useful in hub, repeater, bridge, switch
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  • Migooo77
    Basically, a collision domain is a network segment that allows normal network traffic to flow back and forth. In the old days of hubs, this meant you had a lot of collisions, and the old CSMA/CD would be working overtime to try to get those packets re-sent every time there was a collision on the wire (since ethernet allows only one host to be transmitting at once without there being a traffic jam). With switches, you break up collision domains by switching packets bound for other collision domains. These days, since we mostly use switches to connect computers to the network, you generally have one collision domain to a PC.


    Broadcast domains are exactly what they imply: they are network segments that allow broadcasts to be sent across them. Since switches and bridges allow for broadcast traffic to go unswitched, broadcasts can traverse collision domains freely. Routers, however, don't allow broadcasts through by default, so when a broadcast hits a router (or the perimeter of a VLAN), it doesn't get
    forwarded. The simple way to look at it is this way: switches break up collision domains, while routers (and VLANs) break up collision domains and broadcast domains. Also, a broadcast domain can contain multiple collision domains, but a collision domain can never have more than one broadcast domain associated with it.
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