Collision Domain Vs. Broadcast Domain

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LAYER 1 - PHYSICAL LAYER

Devices - Hubs, Repeaters

Collision Domain : As you might have studied both these devices just forward the data as it is to all the devices that are connected to them after attenuating it ( making it stronger so that it travels more distance ). All the devices fall in the SAME COLLISION DOMAIN because two or more devices might send the data at the same time even when we have CSMA/CD working. So, the data can collide and nullify eachother that way no one gets nothing.

Broadcast Domain : These devices don't use any type of addressing schemes to help them forward the data like MAC or Ip addresses. So, if a PC A sends something for PC B and there are also C,D and E PC's connected to the hub then all the devices i.e B,C,D and E would recieve the data ( Only PC B accepts it while others drop it ). This is what is being in a single BROADCAST DOMAIN.

LAYER 2 - DATA LINK LAYER

Devies : Bridges , Swtiches

Collision Domain : Unlike the layer 1 devices, here, the layer 2 devices break up the collision domain. This means, suppose 4 devices are connected to a 12 port switch. Now, each of these devices are in a seperate collision domain. In our case since we have 4 devices or PC's connected to 4 switch ports then we have here 4 collision domains. The switch being a 12 port one, it can provide us with 12 collision domains. Remember, ONE PORT ONE COLLISION DOMAIN, TWO PORT TWO COLLISION DOMAINS and so on.

Broadcast Domain : Like the layer 1 devices, the layer 2 devices fall into the category of being no able to break the broadcast domain.For example - 4 devices connected to a switch make 4 collision domains like we saw above but all the these devices are still in one broadcast domain, BY DEFAULT. By default means that right out of the box the switch won't divide the broadcast domain, however, you can configure VLAN's in order for the switch to divide the BROADCAST DOMAIN.

All the above applies for bridges as well however, you don't see bridges anymore. Like repeaters were replaced by hubs as the latter was multiport and could support more devices, the same happened with bridges as they were replaced by switches since the latter were multiport.

LAYER 3 - NETWORK LAYER

Devices Used : Routers, Layer 3 switches

Collision Domain : Layer 3 devices are similar to layer 2 ones as they are capable of dividing the collision domain. 2 devices connected to router are in 2 seperate collision domains. Nothing fancy here, quite simple.

Broadcast Domain : Layer 3 devices by default are capable of dividing the broadcast domain. No configuration is required as was the case with switches.

The above scenario when we has 2 devices connected to a router, we get 2 collision domains and 2 broadcast domains.

That was pretty easy. :D

Lets put this in one place :

 

                                                   COLLISION DOMAIN                                    BROADCAST DOMAIN

 

Hub/Repeater                                  Doesn't Split                                                     Doesn't Split

 

Switch/Bridge                                  Does Split                                                   Doesn't Split (By default)

                                                                                                                      Can be done through VLAN

Router                                             Does Split                                                        Does Split

 

Lets see this image :

 

On the left, hub and 3 devices. One big collision domain and one big broadcast domain.

On the right, switch and 3 devices. 3 collision domains and one big broadcast domain.

 

 

 

Let us see one more :

 

[img]http://certforumz.com/index.php?action=dlattach;topic=878.0;attach=3484[/img]

 

Everything has been labelled to facilitate your understanding.

 

Hope all of this helps. Anymore questions, i will be happy to answer.