Collision Domain Confusion?

simzz

Hi,

I am bit confused with collision domain topic. I know what is a collision domain but if i look on different network diagrams i can never tell how many collision domains are there in the diagram.

Actually my concept is not clear. Can some one please explain with some diagrams (if possible) how to find out no. of collision domains//

I wil be really grateful.

thanks

hexem

Depends on the devices, know what a hub and switch look like.

Each port off a switch is it's own collision domain. A hub is a physical multi-port repeater, all hosts attatched are part of a single collision domain.

check out my diagram.

CiskHo

A port off of a router would be a collision domain as well wouldn't it? In particular, what about the serial link between the two routers in your diagram? I would consider the diagram to show 6 collision domains, 1 being the serial link between the routers. However, I wouldn't think the CCNA or CCENT exams would even give a diagram with multiple routers (too advanced) but I could be wrong.

Simmzz: I had issues with the same dollision domain counting questions too. Once I looked at it like this I started understanding it:
Every cable coming off of a hub is part of ONE collision domain (hubs suck so everyone has to share the same domain/wire).

simzz

hexem wrote: »

Depends on the devices, know what a hub and switch look like.

Each port off a switch is it's own collision domain. A hub is a physical multi-port repeater, all hosts attatched are part of a single collision domain.

check out my diagram.

ok so in your diagram how many collision domains are there: i guess 5 in total (4 for switch and 1 for hub). but i saw a very similar diagram to urs where the answer should be 14 but it was 15.. have a look on my example



the actual answer from on online test website is 15 but whereas if i count is 14. pls let me kno wat do u think

networker050184

There are 15 links there, which one are you not counting?

simzz

ok do you count the link between 2 routers as well

CiskHo

simzz wrote: »

ok do you count the link between 2 routers as well

I guess I knew more than I thought! The only way you are able to get 15 collision domains out of that diagram is if you count the link between the routers... and seeing that a router is not a hub I don't see why it would not be counted.

thenjduke

There is only 5 collision domains in this picture. Every Wan connection works at full duplex so would be no collision there.

networker050184

thenjduke wrote: »

There is only 5 collision domains in this picture. Every Wan connection works at full duplex so would be no collision there.

Say what?

CiskHo

networker050184 wrote: »

Say what?

+1 Not sure what he was trying to say there.....

thenjduke wrote: »

Every Wan connection works at full duplex so would be no collision there.

Which is exactly why it is its own collision domain

Top diagram would have 6 collision domains.
Next diagram would have 15 collision domains.

Per Wiki:
"If a group of Ethernet or Fast Ethernet devices in a CSMA LAN are connected by repeaters they will compete for access on the network. This situation is typically found in a hub environment where each host segment connects to a hub that represents only one collision domain and only one broadcast domain. Only one device in the collision domain may transmit at any one time, and the other devices in the domain listen to the network in order to avoid data collisions."
*Keep in mind that a hub is just a multiport repeater.

moss12

guys isn't there 21 collision domains since each ports its own collision domain

thenjduke

networker050184 wrote: »

Say what?

Oops I apologize it is collision domain. I meant to say that. I was talking to buddy of mine since I was finishing up

thenjduke

CiskHo wrote: »

+1 Not sure what he was trying to say there.....


Which is exactly why it is its own collision domain

Top diagram would have 6 collision domains.
Next diagram would have 15 collision domains.

Per Wiki:
"If a group of Ethernet or Fast Ethernet devices in a CSMA LAN are connected by repeaters they will compete for access on the network. This situation is typically found in a hub environment where each host segment connects to a hub that represents only one collision domain and only one broadcast domain. Only one device in the collision domain may transmit at any one time, and the other devices in the domain listen to the network in order to avoid data collisions."
*Keep in mind that a hub is just a multiport repeater.

I meant it was collision domain. I was talking to buddy and not thinking

CiskHo

moss12 wrote: »

guys isn't there 21 collision domains since each non-hub port is its own collision domain

Yes.
*Edited for accuracy/clarification. Even though the hub in the 1st diagram has multiple ports every connection to that hub/repeater would be part of the same (1) collision domain.

hexem

simzz wrote: »

ok so in your diagram how many collision domains are there: i guess 5 in total (4 for switch and 1 for hub). but i saw a very similar diagram to urs where the answer should be 14 but it was 15.. have a look on my example



the actual answer from on online test website is 15 but whereas if i count is 14. pls let me kno wat do u think

If the two router's were connected via cross-over cable through fastehernet or ethernet port then i'd say there was 15..however over a serial link no, csma/cd is specified in 802.3 ethernet standards.

i think he's confused about how effectively collisions are eliminated through the use of full-duplex, so maybe he thinks they are no longer a collision domain at all.


Now shall we play how many broadcast domains? who wan't to say 3? lol!

ConstantlyLearning

moss12 wrote: »

guys isn't there 21 collision domains since each ports its own collision domain

That used to trip me up.

I think of it as every 'link' is a CD.

Link between a switch or router and a hub (including all the other links off the hub) - 1 CD
Link between two switches - 1 CD
Link between two routers - 1 CD
Link between a switch and a router - 1 CD
Link between a switch and an end device - 1 CD
Link between a router and an end device - 1 CD

simzz

ConstantlyLearning wrote: »

That used to trip me up.

I think of it as every 'link' is a CD.

Link between a switch or router and a hub (including all the other links off the hub) - 1 CD
Link between two switches - 1 CD
Link between two routers - 1 CD
Link between a switch and a router - 1 CD
Link between a switch and an end device - 1 CD
Link between a router and an end device - 1 CD

thanks for the answer matey and ending the confused discussion

websponge

thenjduke wrote: »

There is only 5 collision domains in this picture. Every Wan connection works at full duplex so would be no collision there.

?? where did you get 5 from? there are 9 PCs connected to 3 switchs, thats at least 9 collision domains? they arent hubs are they?

mikem2te

websponge wrote: »

?? where did you get 5 from? there are 9 PCs connected to 3 switchs, thats at least 9 collision domains? they arent hubs are they?

Don't worry there has been two different topologies discussed in this thread, the original one had 6 collision domains, the second had 15.

thenjduke

The first topology has six collision domains. I was not talking about the 2nd one.

bcall64

I know this doesn't answer your question but what really helped me with topics like this was CBTNuggets. It clarified the three way handshake with tcp/ip, switch configuration, collision and broadcast domains. With that and Odom's book I was pretty solid across the board. It was almost like being in a classroom.