Collision Domain vs. Broadcast Domain

RZetlin

I have looked over the Tech Exam notes and I'm still not clear what's the difference between the two domain.

Collision domains deals with the signals that crash into each other on a network while broadcast domain deals with what?

Is the broadcast domain higher up in the OSI model than the collision domain?

lordy

A collision domain is a Layer 2 domain where collisions can happen.
The most typical one is multiple PCs being connected to a Hub.

A broadcast domain is a Layer 3 domain where broadcasts can be send from one host to all hosts on that domain.
A typical broadcast is a switched LAN.

Collision domains are broken up by bridges, switches and routers.
Broadcast domains are only broken up by routers.

RZetlin

lordy wrote:

A collision domain is a Layer 2 domain where collisions can happen.
The most typical one is multiple PCs being connected to a Hub.

A broadcast domain is a Layer 3 domain where broadcasts can be send from one host to all hosts on that domain.
A typical broadcast is a switched LAN.

Collision domains are broken up by bridges, switches and routers.
Broadcast domains are only broken up by routers.

What is being broadcast out in a broadcast domain? Something like ICMP?

lordy

ICMP could be one use (e.g. boardcast ping).

The most widely known is probably Windows advertising available shares and printers via broadcast.

xwesleyxwillisx

VLANS created on the switches can also break up broadcast domains

And if I'm not mistaken, collisions are a layer 1 problem. I say this because the collisions occur on the physical medium and not necessarily associated with the appropriate layer 2 devices. Correct me if I'm wrong. Wouldn't be the first time.

Webmaster

RZetlin wrote:

What is being broadcast out in a broadcast domain? Something like ICMP?

Typical examples are ARP and DHCP broadcasts. I.e. when a DHCP client sends out a broadcast (DHCPDiscover packet) to find a DHCP server. Or when a TCP/IP client wants to communicate with another TCP/IP client on the same LAN, it will send out an ARP request broadcast to find out the layer 2 (MAC) address of the target client. These type of broadcasts remain on the same layer 2 segment/LAN. Network nodes in different layer 2 broadcast domains need layer 3 connectivity to be able to communicate with eachother. In reality (and recommended by Cisco), there's a one-to-one relationship between a broadcast domain (LAN or VLAN) and a layer 3 subnet.

VLANS created on the switches can also break up broadcast domains

Indeed. But it would indeed 'break' it up in to separate broadcast domain. You'd need a router (or layer 3 switch) to allow communcation between systems in both broadcast domains.

And if I'm not mistaken, collisions are a layer 1 problem. I say this because the collisions occur on the physical medium and not necessarily associated with the appropriate layer 2 devices.

Also correct. Collisions are caused by multiple systems putting an electrical signal on the same 'wire', which can be extended by hubs or repeaters (which btw both have colissions leds). You can separate collission domains by using a layer 2 (or higher) device. 'Adding another layer'...

lordy

My bad. You are right about VLANs and Layer 1.

Maybe I shouldn't answer these kind of questions after drinking beer