Argh.. Collision and broadcast domain, still confused..

SurferdudeHB

I've reread multiple times and used CBT nuggets but still don't fully understand what is the difference between a collison and broadcast domain. Any sample illustrations would help, thank you.

tha_dub

Basically you need to understand the difference between hubs and switches and you'll get the difference between the two.

Hub: Say you have an 8 port 100meg hub. When any one pc places data on the wire it is automatically sent to all other ports (pc's) on that hub and any other hubs daisy chained to it. This is a collision domain because another PC could place data on the wire at the same time as another and thus cause a collision. It doesn't matter if either pc is sending unicast or broadcast packets everything gets sent to all ports because that is all a hub does. Hubs are also half duplex devices which means data can only from in one direction at a time.

A switch on the other hand can tell the difference between a broadcast and a unicast packet it receives. If an 8 port switch receives a packet from one PC destined for another PC (unicast) it only sends that data down the wire the destination PC resides. This is why it's called a switch. It creates a virtual path between two devices. When a switch receives a broadcast it will forward out all ports except the port it received the broadcast on (no point in echoing back what you were just told) Now the reason this does not cause collisions is because switches are full duplex. They send and receive at the same time so it doesn't matter if a pc receives a broadcast at the same time they are sending data.


For test questions just remember anything connected to a hub is in the same single collision domain and that each switch/router end to end connection is it's own collision domain. Also routers do not forward broadcasts so they separate broadcast domains whereas switches and hubs do not.
HTH

D

IRONMONKUS

http://www.techexams.net/forums/ccna-ccent/19808-collision-domain-vs-broadcast-domain.html

Think of it as an analogy about cars on the freeway.

HUB: (A collision domain)

All computers(cars) connected to a hub share the same freeway and so traffic just flows. Cars can change lanes and maybe even crash into each other causing a collision. Users roll down the car windows and scream at the other cars to find out where they are on the freeway (broadcast)

SWITCH: (Breaks up collision domains)

Each computer(car) on a port(lane) has to stay in it's own lane and is free to just drive and doesn't have to worry about other cars crashing into it. Cars can still see other cars, can roll the windows down and scream at the other drivers to find out where they are and what lane they are in (broadcast).

ROUTER: (Breaks up collision and broadcast domains)

A router is like a roadblock on the freeway, stopping cars that have users that scream out the windows and crash into other cars.

So, a collision domain is a group of cars that can possibly crash into each other and a broadcast domain is a group of cars that can roll there windows down and scream at the other cars to find out where they are and in which lane they are driving in.

Basically:
hubs = one whole collision domain (shared traffic), one broadcast domain
switches = each port is it's own collision domain, one broadcast domain
routers = stop collision and broadcast domains

---

I hope I didn't just make things worse.

phonetic.man

SurferdudeHB wrote: »

I've reread multiple times and used CBT nuggets but still don't fully understand what is the difference between a collison and broadcast domain. Any sample illustrations would help, thank you.

Without getting to specifics and technical details, here is the general idea.

Collision domain - A physical network segment where data packets can collide. Collision domains are broken up by intelligent networking devices such as switches (both layer 2 and layer 3 switches) and routers. Each port on these intelligent devices is its own collision domain. Hubs are dumb devices and only provide more ports for connectivity. A hub shares/multiplies an existing connection with more than one device. Every port on a hub is part of a single collision and broad cast domain. Each port on a switch/router is its own collision domain. Every port on a switch is part of a single broadcast domain.

Broadcast domain - A logical network segment. Broadcast domains are only broken up by devices that operate at Layer 3 (Routers or Layer3 switches). Each port on a Layer3 switch and router can be its own broadcast domain (depending on the device's configuration). The layer3 switch and router knows where a network lives (192.168.1.x lives out port e0 and 192.16.2.x lives out pot e2 etc...)

Jollycork

IRONMONKUS wrote: »

http://www.techexams.net/forums/ccna-ccent/19808-collision-domain-vs-broadcast-domain.html

Think of it as an analogy about cars on the freeway.

HUB: (A collision domain)

All computers(cars) connected to a hub share the same freeway and so traffic just flows. Cars can change lanes and maybe even crash into each other causing a collision. Users roll down the car windows and scream at the other cars to find out where they are on the freeway (broadcast)

SWITCH: (Breaks up collision domains)

Each computer(car) on a port(lane) has to stay in it's own lane and is free to just drive and doesn't have to worry about other cars crashing into it. Cars can still see other cars, can roll the windows down and scream at the other drivers to find out where they are and what lane they are in (broadcast).

ROUTER: (Breaks up collision and broadcast domains)

A router is like a roadblock on the freeway, stopping cars that have users that scream out the windows and crash into other cars.

So, a collision domain is a group of cars that can possibly crash into each other and a broadcast domain is a group of cars that can roll there windows down and scream at the other cars to find out where they are and in which lane they are driving in.

Basically:
hubs = one whole collision domain (shared traffic), one broadcast domain
switches = each port is it's own collision domain, one broadcast domain
routers = stop collision and broadcast domains

---

I hope I didn't just make things worse.

Classic!!! love the broadcast analogy!!!

Priston

IRONMONKUS wrote: »

http://www.techexams.net/forums/ccna-ccent/19808-collision-domain-vs-broadcast-domain.html

Think of it as an analogy about cars on the freeway.

HUB: (A collision domain)

All computers(cars) connected to a hub share the same freeway and so traffic just flows. Cars can change lanes and maybe even crash into each other causing a collision. Users roll down the car windows and scream at the other cars to find out where they are on the freeway (broadcast)

SWITCH: (Breaks up collision domains)

Each computer(car) on a port(lane) has to stay in it's own lane and is free to just drive and doesn't have to worry about other cars crashing into it. Cars can still see other cars, can roll the windows down and scream at the other drivers to find out where they are and what lane they are in (broadcast).

ROUTER: (Breaks up collision and broadcast domains)

A router is like a roadblock on the freeway, stopping cars that have users that scream out the windows and crash into other cars.

So, a collision domain is a group of cars that can possibly crash into each other and a broadcast domain is a group of cars that can roll there windows down and scream at the other cars to find out where they are and in which lane they are driving in.

Basically:
hubs = one whole collision domain (shared traffic), one broadcast domain
switches = each port is it's own collision domain, one broadcast domain
routers = stop collision and broadcast domains

---

I hope I didn't just make things worse.

So, what your saying is:
broadcast domain (router) is like the 2 men on the sides of the road during road construction when there is only 1 lane is available. There is a person on each side directing traffic with there stop/slow.
one collision domain (hub) is like a 4 way stop sign. 4 people arive at the same time and there might be multiple collisions.
separating collision domains in multiple collison domains (Switches) is like a traffic circle. The only time a collision happens is between 2 people.

Jollycork

Priston wrote: »

So, what your saying is:
broadcast domain (router) is like the 2 men on the sides of the road during road construction when there is only 1 lane is available. There is a person on each side directing traffic with there stop/slow.
one collision domain (hub) is like a 4 way stop sign. 4 people arive at the same time and there might be multiple collisions.
separating collision domains in multiple collison domains (Switches) is like a traffic circle. The only time a collision happens is between 2 people.

A switch is like a traffic cop that directs traffic to it's specific destination. The traffic cop knows where each car's destination is [sign on the roof saying New York or bust] and directs the car to it's specific road to reach it's destination.

A hub is like having multiple roads going somewhere, but with no road signs or maps. Cars going somewhere have to go to each destination to find out if it's the one they want. All the while there are other cars on the roads going somewhere and each of them have to take every road to find out if it's the one they want. With lots of cars on the road, going to or coming from the multiple roads, eventually some cars will collide with other cars on the road.

broadcast domains don't really lend itself to the car analogy....

Sorta have to use the people with bullhorns in a room analogy. A router is the room. Everyone in the room has a bullhorn and can "broadcast" to everyone else in the room. No one outside the room can hear the "broadcast".

Priston

o yea, in order for a router to really be in this it'd have to be something like all cars go out this link and all trucks go out this other link right?

I'm starting to get confused myself x.x when I was thinking about it in my head I was thinking the router was an internal router working as a bridge between one network and another

VinUnleaded

From what I understand (I still dont know much), a broadcast domain is when a host sends a broadcast, EVERY host on the network has to receive the broadcast. This generates a lot of traffic. So routers were made to stop the broadcasts from going to other networks.

Imagine the internet with 1 broadcast domain, every computer, every host has to process broadcast messages from trillion of other computers every seconds. There would be no room for real traffic.

So a router acts like a spam filter. Once it receives a spam letter, it wont make duplicates of the letters and send it to every network its connected to

Please correct me if im wrong!

tha_dub

VinUnleaded wrote: »

From what I understand (I still dont know much), a broadcast domain is when a host sends a broadcast, EVERY host on the network has to receive the broadcast. This generates a lot of traffic. So routers were made to stop the broadcasts from going to other networks.

Imagine the internet with 1 broadcast domain, every computer, every host has to process broadcast messages from trillion of other computers every seconds. There would be no room for real traffic.

So a router acts like a spam filter. Once it receives a spam letter, it wont make duplicates of the letters and send it to every network its connected to

Please correct me if im wrong!

yep that's pretty much right. Also keep in mind VLAN's separate broadcast domains as well.