Collision Domains

Unanswered Question
Mar 1st, 2007

I am trying to get a better understanding of collision domains. I understand that if I have a switch with 3 PC's plugged into the switch I have 3 collision domains. What happens if I have another switch plugged into a regular port from the switch and also a router plugged into the switch? Do I then have 5 collision domains or do I still have just the 3? One additional for the other switch and one for the router or do they not count?

I have this problem too.
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Jon Marshall Thu, 03/01/2007 - 15:10


Yes you now have 5 collision domains on your switch. It's not so much what is connected into each port it is more to do with each port creating a private collision domain between itself and whatever is connected into it.



bmellon0720 Thu, 03/01/2007 - 15:20

That helps, thank you. I have one additional question about broadcast domains. I have 3 routers, we will call them router A, router B and router C. The routers connect to each other. Router A has a switch and 2 PC's, router B has a switch a PC and a printer and router C has a switch and 3 PC's. All of the routers are linked together. Would I have 3 broadcast domains? Would I also be correct when saying that I have 3 collision domains for router A, 3 collision domains for router B and 4 collision domains for router C?

Francois Tallet Thu, 03/01/2007 - 15:28

I don't know what you are looking for counting this way collision domain and broadcast domain. Generally speaking, a broadcast domain is where a broadcast would be flooded at layer 2. In your network, if all those devices are sharing a single vlan, that would be one broadcast domain. As far as collision domains are concerned, now that most devices (including the cheapest pcs) are able to run full duplex with your switch, I'm not sure this is even relevant in your network.




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