Collision domains

Jasiono

Reading this book, I stumbled across my confusion again that haunted me in the past in regards to collision domains.

I know switches break up collision domains within broadcast domains.

Say that I have the following setup:

Switch A
Switch B
Switch C
Router 1
PC1
PC2

Router 1 is connected to Switch C
Switch C connects to Switch A and B
PC1 is connected to Switch A
PC2 is connected to Switch B

Does this mean I have 5 collision domains?

1 for the connection between the router and switch C
1 for the connection of switch C being connected to switch A
1 for the connection of switch C being connected to switch B
1 for the PC being connected to Switch A
1 for the PC being connected to Switch B

So basically, to my understanding now, is that each PORT on a SWITCH creates a collision domain. Is that a safe assumption?

d4nz1g

Yes, each port on a switch is a collision domain.

In theory, every shared medium (like a hub, for example) is a collision domain.

Regarding to broadcast domains, each VLAN represents a broadcast domain since a broadcast is destined to every host connected to a given switch (or vlan). So in a switched environment, you will have a broadcast domain for each vlan.

Jasiono

Ok, so based off of what you said and searching further into it, finding out that routers also break up collision domains, I attached a diagram I quickly put together and came up with the following:

24 Collision domains
8 broadcast domains

Is that correct?

d4nz1g

20 collision domains, actually.
The collision concept only exists in ethernet.

Jasiono

Ok so basically what is being said here is that if a router is connected to another router, only count that as a broadcast domain and not a collision domain because it's not connected to an ethernet port on a switch, yeah?

That is where my confusion comes in.

A switch to a router is a collision domain but a router to a router is not.

Thanks for the time in explaining this to me, I really appreciate it.

tecnodog7

d4nz1g wrote: »

20 collision domains, actually.
The collision concept only exists in ethernet.

Ok i counted wrong, yes it's 20 :P

Jasiono

Do you count a router connection to an ISP a broadcast domain?

theodoxa

tecnodog7 wrote: »

Ok i counted wrong, yes it's 20 :P

I'm counting 21.

Jasiono

Circled in red is what I counted as a collision domain.

seigex

Jasiono wrote: »

Circled in red is what I counted as a collision domain.

Unless I'm missing something, bridges are L2, and separate collision domains. So it would be 21 by my count

Jasiono

Oh yeah, it IS 21 lol. I did mean to put in a hub instead.

Ugh

tecnodog7

theodoxa wrote: »

I'm counting 21.

Damn you and your counting skills .

Yes it is 21 since there's a Bridge in the middle which i mistook for a HUB. Doh!!!!!!!

sendalot

Is your inter-switch connection on access or trunk mode?

Jasiono

I just did connections, nothing radical or fancy. There is nothing configured on anything.