Collision and broadcast domain practice problems

[Deleted User]

Hey forum,


I'm in the middle of my CCENT studies and I'm trying to find some collision and broadcast domain practice problems. That got me on my last attempt and probably kept me from passing. Anyways does anyone have any practice questions for this? Thanks.

Codeman6669

what is it that you find challenging about the questions? is there anything you dont understand or know about collision/broadcast domains?

cisco_kidd20

You may just need to understand the concept of what a collision domain is and what a broadcast domain is. It's not really a chapter topic, more like a few pages, or...just the link below.

Collision Domains vs. Broadcast Domains « Cisco Skills

theodoxa

A Collision domain is where a collision could occur. That is a shared medium. In the "olden" days there were two things you might have seen --

1. Hubs used in the Access Layer instead of switches. Switches were extremely expensive not all that long ago. A Hub is a Layer 1 "dumb" device which simply repeats anything it hears on one port out all other ports.

2. ThinNet or ThickNet Based Ethernet. Originally Ethernet used Coaxial cable. All the PCs on a LAN would simply be connected to this one cable using either an adapter or what was called a vampire tap. Because all the devices had to use the same cable, it was a shared medium.

In either case, if two devices on the same shared medium (either connected to the same cable or to a hub) were to transmit at the same time, their frames would collide resulting in a collision. CSMA/CD (Carrier Sense Multiple Access/Collision Detection) was developed to reduce the likelihood of this by checking the line before transmitting and if a collision was detected by "backing off" and waiting a random period of time to retransmit the frame. This still didn't prevent all collisions because it was possible for 2 devices to attempt to transmit at the same time.

Think of an analog home phone (if you're old enough to remember them). You could pick up one phone and hear what someone was saying on another phone in the same house. Within your home, they were on a shared medium. If you picked up the phone and heard let's say your mother talking (mine seemed to always be on the phone growing up), you hung up. This is CSMA. On the other hand, let's say you picked up the phone and didn't hear anyone, so you started to dial, but at that exact same time your mother picked up the phone and having not heard anyone speaking also started to dial (this actually happened occasionally). The result is a collision. CD would mandate that both of you hang up and wait a random amount of time before attempting the call again. In our house, it worked a little different.

Another way to think of it is like when your mother told you to get your father for dinner. A hub would just yell "Dad, Dinner's Ready!" at the top of its lungs. A switch would actually go and tell its father face to face. The first option results in everybody in the house hearing and while you're yelling it is impossible for any other conversations to occur. The second delivers the message only to the intended recipient (obviously you would have to know where your dad is to go to him) and allows other people in the house to have their own conversations at the same time.

Nowadays, there is really only one device you will frequently run into that will result in multiple devices in a single collision domain and that is a Wireless Access Point. This is because the air is a shared medium. For modern wired networks, the collision domain exists only on the cable between the PC and the switch.

A broadcast domain is where a broadcast will be forwarded. This is the Layer 2 portion of your network. If a device sends a broadcast, it will be forwarded across the Layer 2 portion of the network to every device. A Layer 3 device will not forward a broadcast by default into a different LAN/VLAN.

[Deleted User]

I understand how they work. I'm just looking for problems because on my last exam due to speed I may have overlooked a CD. That's all. Each time I have taken the exam I only had around 20secs left. I just need to speed up the CD and BD process to have time for sim questions.

bharvey92

Imagine you have a LAN, you have 1 switch and 10 hosts are connected to the switch, how many collision domains are there? You then add a router, how many broadcast domains are there? (there are no VLANs implemented in this design).

[Deleted User]

Well for each port on a switch is a collision domain so 10 Collision Domains. If the router is only using 1 port that is connected to the switch that is 1 Broadcast Domain.

cisco_kidd20

You get the concept, what are you having trouble with? Questions like that one is what you will see on the test.

bharvey92

cisco_kidd20 wrote: »

You get the concept, what are you having trouble with? Questions like that one is what you will see on the test.

Cisco Kidd is correct, they are basic CCNA questions so you sound like you got the basics covered. Just remember that each VLAN is it's own Broadcast domain.

Jon_Cisco

Don't forget Vlans when working with you broadcast domains.

As to the OP question I did not come across any site specifically designed to practice this.


edit: I'm not sure if any of the broadcast domain questions had anything about vlans I can't really recall any. But I know when seeing what a switch does with a frame you need to keep an eye on the vlans.

theodoxa

Jon_Cisco wrote: »

edit: I'm not sure if any of the broadcast domain questions had anything about vlans I can't really recall any.

The CCENT (ICND1) and CCNA (ICND2) have been revised since I took them, but when I took ICND1, you were not supposed to know about VLANs yet. I remember looking at practice questions and thinking "How many VLANs does the switch have configured?"