Collision domain clarification

tomrandz

Hey guys,

I'm completing my latest computer networking assignment and have a task to count the number of broadcast and collision domains within a few different examples. I feel comfortable doing this but I'm unsure about a small section of it. Just wanted some clarification if I have got the right idea, any help would be appreciated.

The part I'm unsure about is the top right hand side where there is 2 switches connected to the same hub, do I still count that as one collision domain or do I count it as 2?



Also if there's any I've missed, feel free to point them out.

Tom

vinbuck

Think about it this way...if PC17 sends a frame, which port or ports on HUB5 will send the frame out?

tomrandz

It will send out on all ports?

tomrandz

Can anyone clarify this for me?

Tom

ptilsen

Go back to the OSI model. What separates collision domains? Only some form of layer-two device. There is no layer two device separating the PC and two switches from each other.

sizeon

4 collision domains and 1 broadcast domain for the LAN at the top right hand corner.

ptilsen

sizeon wrote: »

4 collision domains and 1 broadcast domain for the LAN at the top right hand corner.

No, OP is not referring the the four PCs and switch in the upper-right. He is referring to the hub below that switch which is connecting those two switches and a PC.

sizeon

oh, that would be 1 collision domain and 1 broadcast domain

Todd Burrell

Maybe I'm trying to make this too simple, but the way I remember it is that routers and switches via VLANS break up broadcast domains. Routers, switches and bridges break up collision domains. And remember that links between these devices will also count as a 'domain'. So if you connect 2 switches directly that is a collision domain.

Back to the picture - the portion where you have 2 switches connected with a hub in the middle would all be one collision domain - and it would be part of a much larger broadcast domain.

instant000

The easiest way to look at it is this:

1. Always ignore a hub. it provides no function of breaking up anything. It just repeats everything it hears.
2. Every port on a switch is on a different collision domain
3. Every port on a router is on a different broadcast domain AND different collision domain.

If someone starts bringing up VLANs, then those are broadcast domains, but you won't likely encounter a tricky query like that. However, in a straight forward diagram with layer 1 (hub), layer 2 (switch) and layer 3 (router) devices, you should be able to know what's going on with the collision and broadcast domains.

Hope this helps.

Forsaken_GA

sizeon wrote: »

4 collision domains and 1 broadcast domain for the LAN at the top right hand corner.

No, there would be 5. Each PC 12 through 15 is it's own collision domain, and the link back to Hub5 is an additional collision domain, which also includes PC17 and the other switch that's attached to the Hub.

sizeon

i was talking about the LAN connected to the switch not the hub.