PDA

View Full Version : How to Calculate No. Of Broadcast & Collison Domain In a Network?


Bruce
12-27-2007, 12:57 PM
Hello Sir,

I have recently bought your book about CCNA guide for 640-802 and i feel good while studying , its an friendly guide.

I've started using it by today itself and i got an doubt into the first chapter.

How did u calculated a No.Of Broadcast and Collison domain in an network with Switches, Hubs , Bridges & Router ?

I have tried to get through them, but could not find an good solution.
So, thought to go through your great site and ask for help.

You can view it like this , that i want to understand :

How to find No. Of broadcast & Collison domain in a networking using Only switches ?

How to find No. Of broadcast & Collison domain in a networking using Only Hubs ?


How to find No. Of broadcast & Collison domain in a networking using Switches, Hubs , Bridges & a Router ?

I am hoping to solve my doubt soon and carry forward to the next level.:p

Best Regards

dbeare
12-31-2007, 12:50 PM
Hubs:
Hubs are simple, they do not break up collision or broadcast domains, they simply retransmit the signal to all ports.

Switches:
At this point, we will only concern ourselves with a non-VLAN environment. Each switch port is its own collision domain, but all ports attached to a switch are 1 broadcast domain. Once VLAN's are introduced, you will see how they actually break up a switch into separate broadcast domains, but for right now, only concern yourself with a switch being a single broadcast domain.

Bridges:
Just like switches, except they usually don't have nearly as many ports. Bridges break up collision domains, but do not break up broadcast domains and will forward broadcasts to all hosts.

Routers:
Routers are concerned with breaking up broadcast domains. You can look at each port off of a router as an individual broadcast domain.

Some of these things will change once other technologies are introduced later in your studies, but for now, this is what you will want to know. Hope this helped!

xpaslan
01-26-2010, 07:44 AM
Hubs:
Hubs are simple, they do not break up collision or broadcast domains, they simply retransmit the signal to all ports.

Switches:
At this point, we will only concern ourselves with a non-VLAN environment. Each switch port is its own collision domain, but all ports attached to a switch are 1 broadcast domain. Once VLAN's are introduced, you will see how they actually break up a switch into separate broadcast domains, but for right now, only concern yourself with a switch being a single broadcast domain.

Bridges:
Just like switches, except they usually don't have nearly as many ports. Bridges break up collision domains, but do not break up broadcast domains and will forward broadcasts to all hosts.

Routers:
Routers are concerned with breaking up broadcast domains. You can look at each port off of a router as an individual broadcast domain.

Some of these things will change once other technologies are introduced later in your studies, but for now, this is what you will want to know. Hope this helped!
This is definitely the right answer.
Thank God for people like you!!!

Fuzz
01-26-2010, 11:32 AM
Simply put:

Broadcast domain = subnet
Collision domain = single full duplex link

ladykatherine
11-18-2010, 11:32 PM
hello,

I just started my CCNA 1 class today and I got confused with Broadcast and Collision domain..Is there anyone here who can explain to me on how to count broadcast domain and collision domain in a certain network?:confused:

Broadcast domain refers to all PCs in the network? Do I have to count the hub, server, switch, and router as one of the broadcast domain? How does it work.:confused:

milanchatterjee
11-19-2010, 11:22 PM
Hi katherin
First you have to understand the difference between a Router and a Switch. basically they are the same and they have some (not all) similar functionalities.

As you may have read, that the devices which we will know as Switches resides in Layer 2 and the devices, which we know as Routers resides in layer 3 of the 7 point layer of the network world.

Now, please understand that Routers break up Boradcast domains and Switches break up collision domains.

Understand the scenario. The head router Chief-Vitalstatistix is connected to three routers Asterix, Obelix and Getafix.

Now, from Asterix router, you have 5 networks connected to five switches like Fullyautomatix, Unhygenix, Cacophonix et all.

Similarly, router Obelix have three networks connected to three Switches like Wild-Boar, Roman-Thumping, Dogmatix et all and router Getafix is connected to one network only called magic-Potion

Now, the router Vitalstatistix, which is at the top of "our" pyramid is connected to three more routers at the 2nd rung. That means, that he is breaking up one Broadcast domain into three BC domains. viz routers Asterix, Obelix and Getafix.

Now, Asterix is connected is connected to five switches, and each of them may be connected to five more switches. That implies that the switches are breaking up the collision domains assigned to them, but all of them are under ONE BROADCAST DOMAIN, of Asterix router, who inherited a broken-up broadcast domain from Vitalstatistix.

I hope that I had been clear with you. If any further clarification, please fell free to ask.

Milan Chatterjee

ladykatherine
11-20-2010, 01:23 AM
Hello,

Thanks Milan for replying... Yup I know the diff. between switch and router. I'm sorry I'm not familiar with Fullyautomatix, Unhygenix, Cacophonix, Asterix, Obelix and Getafix. What I know is CISCO routers and LINKSYS routers.

Anyway, all the PCs connected to a router is considered as 1 BROADCAST DOMAIN? and another network with PCs connected to a another router is counted as 1 BROADCAST DOMAIN? So if there are two routers, there are 2 BROADCAST DOMAIN? Therefore, I have to count the number of routers in the network to know the number of BROADCAST DOMAIN? :confused:

Regarding with COLLISION DOMAIN, all the PCs connected to a switch is considered as 1 COLLISION DOMAIN? So that means I just have to count the number of switch in the network to know the number of COLLISION DOMAIN?:confused:

milanchatterjee
11-20-2010, 03:16 AM
Hello,

Thanks Milan for replying... Yup I know the diff. between switch and router. I'm sorry I'm not familiar with Fullyautomatix, Unhygenix, Cacophonix, Asterix, Obelix and Getafix. What I know is CISCO routers and LINKSYS routers.

HI
Sorry. I actually name my routers in my simulator after my favorite Comic characters.

Anyway, all the PCs connected to a router is considered as 1 BROADCAST DOMAIN?

True.

and another network with PCs connected to a another router is counted as 1 BROADCAST DOMAIN? So if there are two routers, there are 2 BROADCAST DOMAIN?

True.

Therefore, I have to count the number of routers in the network to know the number of BROADCAST DOMAIN? :confused:

Regarding with COLLISION DOMAIN, all the PCs connected to a switch is considered as 1 COLLISION DOMAIN?

True.

So that means I just have to count the number of switch in the network to know the number of COLLISION DOMAIN?:confused:


Milan Chatterjee

ladykatherine
11-20-2010, 01:56 PM
ok.. So what I have said are all correct. Now, how about the serial links. I've heard so many things in other forums and other pages in this LAMMLE forum that most of them are confused with regards to counting collision domain in serial links.

What is it all about?:confused:

Fuzz
11-22-2010, 06:38 AM
Serial links are point to point so they are a single collision domain.

ladykatherine
11-23-2010, 12:16 AM
Thanks Fuzz. How can I identify if it is a serial link. Is there any icon designated for serial link or just devices connected directly to each other are called serial link?:confused:

Sorry if I have so many questions. Just want to ask all these questions in my mind to get better understanding with networks.

Big Evil
11-23-2010, 09:49 AM
I do not think there is an Icon, if it is a diagram it should have the link shown as "S" followed by the slot number say, S0/1.

HTH.

bs_kwaj
11-23-2010, 11:44 AM
Sorry if I have so many questions. Just want to ask all these questions in my mind to get better understanding with networks.

Might I suggest a good place to start:

http://www.amazon.com/dp/0470110082?tag=httpwwwglob05-20&camp=14573&creative=327641&linkCode=as1&creativeASIN=0470110082&adid=1E1375QWXK368VKV3CEX&

:)

Fuzz
11-24-2010, 07:14 AM
On network diagrams, serial links are usually represented by a jagged line. Ethernet links are straight and right angled.

http://lh6.ggpht.com/_KFnl8FWE-Rw/SiElzH3wpcI/AAAAAAAAAYQ/Nl0n7-nIozk/640-816%20(13).jpg

Serial links connect to serial interfaces, designated in IOS as serial 0/0 etc. A point-to-point ethernet connection is not a serial link, as it could potentially be a multipoint connection if you add a switch on the other end.

Be careful when counting broadcast domains by the number of routers in the network. VLANs create separate broadcast domains on switches, so it is possible to have multiple broadcast domains without any routers (although it would be failry pointless to have such a network.) A broadcast domain is defined by the extent at which a broadcast packet reaches. Routers don't forward broadcast packets by default, so they are considered the boundary. Similarly, a switch wont forward a broadcast from one VLAN to another, so each VLAN is a broadcast domain.

ladykatherine
11-24-2010, 09:28 PM
Thanks Fuzz for the illustration... We haven't discussed the VLAN yet. So it's pretty new to me.


Regarding with broadcast domain, did you mean that if ever there is a switch in a network it is also counted as 1 BROADCAST DOMAIN?:confused:

Based in your illustration, there are 3 BROADCAST DOMAIN... I f ever there is 1 SWITCH between the hosts and the 3 routers, the total BROADCAST DOMAIN is 6?

ladykatherine
11-24-2010, 10:46 PM
Hello Fuzz, I have attached a network sample illustration. I made it myself using paint app. In this illustration, there are 4 SWITCH(es), 2 HUBs, and 3 ROUTERs. Am i just going to count the number of routers and switches in the network to know the number broadcast domain? And am I just going to count the number of hubs in the network to know the number of collision domain?
I fthat's the case, there will be 2 collision domain and 7 broadcast domain?:confused:

Is there anyone here who can figure out if I am understanding the concept of broadcast domain and collision domain correctly?

DonB
11-25-2010, 01:53 AM
Wrong! You cannot just count the devices. You must understand what collision and broadcast domains are and how each device handles them. Switches will break up collision domains but not broadcast domains. Routers will break up both collision and broadcast domains. Hubs will not break up anything.

So in the image you have 17 collision domains and 7 broadcast domains.

Lets look at your image starting at the top.

On the left top you have a switch with 3 pc's and it connects to a router. Each port on a switch is its own collision domain so you have 4 collision domains. Switches do not break up broadcast domains so you have 1 broadcast domain.

On the top right, you have a Hub connected to a switch. The Hub breaks up nothing so you have 1 collision and 1 broadcast domain on the hub. You have 1 additional port connected to a Router which adds another collision domain.

The routers breakup both collision and broadcast domains. Each of the links between the routers count for both a collision and broadcast domain so you have 3 collision domains and 3 broadcast domains here.

The switch at the lower left has 3 pc's and 1 link to the router. This means that you have 4 collision domains and 1 broadcast domain here. Remember switch breakup collision domains but not broadcast domains.

At the lower right you have a switch with 2 pc's and 1 link to the router and 1 link with a hub and 2 pc's. The switch is using 4 ports so there are 4 collision domains and 1 broadcast domain. Remember the hub with 2 pc's count as 1 collision domain and 1 broadcast domain because the hub does not breakup either collision or broadcast domains.

Hopes this clears it up. 17 collision domains and 7 Broadcast domains

DonB
11-25-2010, 02:02 AM
Remember the hub with 2 pc's count as 1 collision domain and 1 broadcast domain because the hub does not breakup either collision or broadcast domains.


I need to clarify the above statement. Since the hub does not breakup collision or broadcast domains, the hub and the 2 pc's are all in 1 collision domain and does not add another broadcast domain. Rather it is part of the single broadcast domain of the switch.

Fuzz
11-25-2010, 07:39 AM
Hello Fuzz, I have attached a network sample illustration. I made it myself using paint app. In this illustration, there are 4 SWITCH(es), 2 HUBs, and 3 ROUTERs. Am i just going to count the number of routers and switches in the network to know the number broadcast domain? And am I just going to count the number of hubs in the network to know the number of collision domain?



No, this is the point I was trying to illustrate. You cannot use devices to work out collision and broadcast domains. As I said, switches can have multiple VLANs, and each VLAN is a broadcast domain. Without knowing the logical topology, you cannot determine the broadcast domains - it is not a physical layout.

What you need to understand is what the terms broadcast and collision domains mean. If I send a packet with destination 192.168.0.255/24, I am sending to the broadcast address for that network. This means that all devices on 192.168.0.0 will look at the packet. Those devices are the broadcast domain.

A collision domain is anywhere there could be more than one device using the same medium. In a full duplex point-to-point link (switch to PC, for example) there can only be two devices, so it is a single collision domain. In a half-duplex hub link, you can have multiple devices all on the same physical layer. This is also a single collision domain.

ladykatherine
11-25-2010, 07:59 AM
Hello,

Thanks DonB, thanks Fuzz... :o

So, if there is a hub in a network it is counted as 1 collision domain (no matter how many PCs are connected to the hub) but the count doesn't stop there, if there is a switch it is also counted as a collision domain. Why does DonB count it as 4 collision domains. Is he counting the number of ports in the switch?

Guess I am not really familiar with the logical topology... because I am just counting the links connected to the router. I am thinking that that would be the number of broadcast domain. And thinking that the number of hubs in the network and the number of ports in the switch will tell the number of collision domains. That's what I am thinking right now.

Is there anyway, such as a tip or technique, not to mess up with this kind of problem (counting the number of collision of domain and broadcast domain in the network)?:confused:

Fuzz
11-25-2010, 10:51 AM
Routers break up broadcast domains because each interface is in a different network, therefore broadcasts don't go beyond the router interface.

Switches break up collision domains because each port allows full-duplex communication. A 24-port switch has up to 24 collision domains. A 24-port hub has one collision domain.

So counting the broadcast domains, count the number of links on all routers. Count the number of links on each switch for the collision domains and forget hubs altogether, as they break up neither. For example, if you have a hub connected to a switch, and PCs connected to the hub, that's one collision domain. Don't count one for the hub and another for the link to the switch.