Collision Domains vs. Broadcast Domains

Fizz

I understand the difference between Collision Domains & Broadcast Domains, my problem is that I can't figure out how to count them in a network. Would someone please post a diagram and illustrate in details the best way to count them!!! :

Thanks in advance.

NeonNoodle

If you have a hub with connections to three PCs, how many collision domains are there? How many broadcast domains are there?

If you have a switch with connections to three PCs, how many collision domains are there? How many broadcast domains are there?

If you have a router with connections to three PCs, how many collision domains are there? How many broadcast domains are there?

How many broadcast and collision domains are in these networks? Think about why the answers are what they are.



How many broadcast and collision domains are in this network? The dotted lines are crossover connections.

Fizz

Thanks for the prompt response. I still can't tell exactly how do you count the number of collision domains since all the networks in the diagrams you provided have the same number of collision domains = 3.
In the network with the HUB it is obvious.

In the network with the HUB and BRIDGE, is it the number of connections to the BRIDGE that equals the number of collision domains????

And does the same apply for the SWITCH and ROUTER networks??

Sorry, I am new to this networking stuff.

Thanks again.
It is really great to be able to come here and have this type of assistance.

r_durant

Fizz wrote:

In the network with the HUB and BRIDGE, is it the number of connections to the BRIDGE that equals the number of collision domains????

And does the same apply for the SWITCH and ROUTER networks??

To answer your questions straight off, yes...

Remember, each connection to a switch, bridge or router interface creates a single collision domain. However, only a router or a vlan breaks up a broadcast domain.

So if you have a 24-port switch with a connection to each port, you have 24 collision domains, but one broadcast domain. Switches or bridges do not break up broadcast domains.

If you have a hub connected to a switch port, all the connections to the hub and the connection from the hub to the switch will be in a single collision domain.

Plazma

think of it like this

collision domains are individual segments of a network in a sense , switches increase the number of collision domains while hubs simply increase the size

each port on a switch is one collision domain and is dedicated to that host on that port so that way nothing else can interfere with the host and the switch.

broadcast domains are individual segments of a network in a sense that broadcasts flow and ONLY can be broken up/divided by layer 3 device, most commonly a router.

if a switch on one network was trying to find the MAC address of a host on another network and it didnt have hte entry in its mac address table , it would broadcast for this address BUT it would stop at one of the routers ports because routers seperate broadcast domains and from there the router will compare the destination IP address to its routing table and send it out the appropriate port.. so each port on a router (ethernet, fastethernet, or serial) will break up broadcast domains.

I think its kinda of a repeat of what was said, but thats my 2 cents, hope it helps.

networker050184

broadcast domains are individual segments of a network in a sense that broadcasts flow and ONLY can be broken up/divided by layer 3 device, most commonly a router.

Layer 2 devices can break up broadcast domains using VLANs. Each VLAN is its own broadcast domain. So you can create multiple broadcast domains with a layer 2 device.

Pash

networker050184 wrote:

broadcast domains are individual segments of a network in a sense that broadcasts flow and ONLY can be broken up/divided by layer 3 device, most commonly a router.

Layer 2 devices can break up broadcast domains using VLANs. Each VLAN is its own broadcast domain. So you can create multiple broadcast domains with a layer 2 device.

Yeh this is something that took me a while to understand. The standard router on a stick setup can have you confused when learning broadcast domains and collision domains. As networker says, vlans segment networks, this is your broadcast domain. Best way to look at is, if a device ARP's, its an all F broadcast, this broadcast isnt gonna pass the router as it's a broadcast for that network ONLY.

Cheers,

Cucumber

To summarize it a little further

If you see a HUB, that counts as one collision domain no matter how many ports the HUB has.

If you see a switch, each port counts as one collision domain

If you see a router, each port counts as one broadcast domain and as one collision domain

You just have to be carefull not counting twice, if you see a router connected to a switch that counts as one collision domain. If a HUB is connected to a switch or router, that counts as one collision domain only.

networker050184

Several broadcast domains can exist connected to a single router port by the use of VLANs.

12beatechie

Cucumber wrote:

If a HUB is connected to a switch or router, that counts as one collision domain only.

What if 2 hubs are connected to a router on 2 ethernet ports, does it count as 3 or 4 collision domains?

NeonNoodle

12beatechie wrote:

Cucumber wrote:

If a HUB is connected to a switch or router, that counts as one collision domain only.

What if 2 hubs are connected to a router on 2 ethernet ports, does it count as 3 or 4 collision domains?

No matter how many hubs you have off of an ethernet interface, you have one collision domain. So, in this case there will be two collision domains.

12beatechie

NeonNoodle wrote:

How many broadcast and collision domains are in this network? The dotted lines are crossover connections.

I want to make sure I nail it down. For the diagram above, the answer is 2 broadcast domains and 8 collision domains.


I came up with 3 different scenarios:

1. A router is connected to 2 switches and each switch consists of 2 PCs.
- Broadcast domains = 2 , collision domains = 6

2. A router is connected to 1 switch and that switch is connected to another switch with 2 PCs on it.
- Broadcast domains = 1 , collision domains = 4

3. A router is connected to 1 switch and that switch is connected to a hub with 2 PCs on it.
- Broadcast domains = 1 , collision domains = 2

Am I correct?

Thanks in advance!

NeonNoodle

You are correct, sir.

akshay nandanwar

can somebody tell me?? How many broadcast domains and collision domains are there?
Thanks in advance

boredgamelad

Tell us what you think the answer is and why you think that's the answer. That way, we can not only give you the right answer but find any flaws in your reasoning.

theodoxa

NeonNoodle wrote: »

8 Collision Domains and 2 Broadcast Domains (assuming that no extra VLANs are configured on the Switch).

Anything connected to a hub, repeater, or some old bus type media (10Base2, 10Base5, etc...) is a single Collision Domain. The entire top half of the diagram is one Collision Domain as everything is connected with hubs. The bottom left is also 1 Collision Domain as it is connected to a hub. Each of the remaining 6 ports on the switch are their own Collision Domain. Collision domains are generally separated by Switches or Bridges (a bridge is just a switch with 2 ports).

By default, all collision domains connected to a switch or bridge are in a single broadcast domain. Therefore, the top half is one broadcast domain and the bottom half another. Broadcast domains are generally divided by routers. VLANs (an ICND2 topic) are the one hitch in this as they allow you to divide a broadcast domain into multiple broadcast domains or VLANs.

theodoxa

NeonNoodle wrote: »

To Elaborate on this diagram...let's consider a few scenarios:

#1 PC1 pings PC3. PC1 sends a frame (or more accurately bits since a hub works on Layer 1) to Hub 0. Hub 0 repeats it out all its other ports, including the port to Hub 1. When Hub 1 receives it, it repeats it out all its other ports also including the port to Router 0. When Router 0 receives the frame, it looks to see if the packet inside is destined for another subnet and if that subnet is in the routing table, it would forward it to that subnet. Since, PC3 is on the same subnet as PC1, the router simply drops the frame/packet as do all the other computers that received it other than PC3.

#2 PC1 pings PC12. This is the same as #1, except that PC12 in on a different subnet. Once the frame reaches the router, the router would deencapsulate it to the packet contained inside and then forward it out Port 1 towards Switch 0. When Switch 0 receives it, it would forward it to Hub 2, who would then repeat it out all of its other ports, including the one PC12 is attached to.

#3 PC12 pings PC23. PC12 sends a frame (bits) to Hub 2, which repeats it out all of its other ports including the one to Switch 0. When Switch 0 receives it, it forwards it out only the port that PC23 is attached to.

#4 PC23 pings PC20. PC23 sends a frame to Switch 0 and Switch 0 forwards it out only the Port PC20 is attached to.

All of this ignores the Flooding which takes place if a switch doesn't have the destination's MAC Address in its MAC Address Table. If a switch doesn't know which port the destination is connected to, it will act more like a hub and simply forward it out all the other ports. Once it gets a reply from the destination, it will add it to its MAC Table. After that, it will only forward it out the port listed in its table for that MAC Address.

sagarrai000

If I'm not wrong,

BD=2
CD=6

Hit me if I'm wrong...

makafui

Can someone please help with the total number of new broadcast domains created if the size of the Broadcast Domain must be reduced such that each concentration of PCs (i.e. Hub 0, Hub 1, Hub2 & Switch 0) is a broadcast domain. Consider your implementation carefully to avoid unnecessary incorporation of equipments, thereby reducing cost of implementation. Thanks in advance.