Calculating Collision Domains

[KarlNey]

This is good stuff, but I don't see how you get 10 collision domains in figure 1-5. I only see 6. Unless you are presuming the first switch is a 5 port switch. Is this correct or am I missing something.

Thanks!

[lammle]

Yes, look again, there are 10 collision domains in figure 1.5.

Do you see nine in figure 1.4?

Each link in figure 1.5 is a collision domain because every link shown in the figure is connected into a switch.

Cheers!
Todd Lammle

[KarlNey]

The light just came on. Thanks!

[BlackJackiv2008]

Sorry Mr Todd Lammle,

I have the same confusion in the Figure 1.4.

From the Note in Chapter 1(Pg8):
Switches create separate collision domains but single broadcast domain.
Routers provide a separate broadcast domain for each interface.

It is easy to count the broadcast domains in Figure 1.4. However it is still confusing to count the collision domain:
Top of the router: I can only see the bridge break the collisions domain into 2.Where would the 3rd collision domain be?
Bottom of the router: 1 collision domain.
Left of the router: Each switch breaks the collision domain, so, only 4 collisions. Where would the 5rd collision domain be?

Hope you can enlighten me. :)

[suneel gupta]

Sorry Mr Todd Lammle,

I have the same confusion in the Figure 1.4.

From the Note in Chapter 1(Pg8):
Switches create separate collision domains but single broadcast domain.
Routers provide a separate broadcast domain for each interface.

It is easy to count the broadcast domains in Figure 1.4. However it is still confusing to count the collision domain:
Top of the router: I can only see the bridge break the collisions domain into 2.Where would the 3rd collision domain be?
Bottom of the router: 1 collision domain.
Left of the router: Each switch breaks the collision domain, so, only 4 collisions. Where would the 5rd collision domain be?

Hope you can enlighten me. :)

Just remember that every port on a bridge or switch is a separate collision domain. So the ports connected to the router are also collision domains.

[darshana]

please explain me in detail .

I still didnt understand how to calculate the collission domains

[Fuzz]

Hub: One collision domain, regardless of how many devices are connected.
Switch: One collision domain for each device connected.

Devices can be PCs, routers or other switches, so remember to count each link as a separate collision domain on a switch or router.

[darshana]

what is it in case of bridges?
how do we calculate the collission domains?
in this case with ref to fig 1.4 of the 6th edition todd lammle there should be 10 collission domains.
but the book reads that the bridge network on the top has three collision domains.

please help me clear my confusion

[Fuzz]

Bridges are the same as switches when it comes to collision domains - one for each link connected to the bridge. In fact, a switch is essentially a multiport bridge.

[Laszlo]

Hi, I started reading the 5th edtion deluxe-version, and I also have a question/comment to ch1.

Figure 1.4 the text bellow says "Also, the bridge only created two collision domains.." Isnt it 3 instead? Later the calculations are correct and used 3.

I like really like the style of the book so far, it almost feels as I am listening to a speaker in the classroom.

Thanks

Laszlo

[Michael950]

I think part of the confusion for figure 1.4 (Hub, Switch, and Bridge) can be contributed to a conflicting count of collision domains:

Source: CCNA Study Guide - 6th Ed.
Figure 1.4 on page 9

On the top internetwork in Figure 1.4, you’ll notice that a bridge was used to connect the hubs to a router. .... Also, the bridge only created two collision domains, so each device connected to a hub is in the same collision domain as every other device connected to that same hub. ....Page 10:

The all-hub network is one collision domain; the bridge network equals three collision domains. Add in the switch network of five collision domains—one for each switch port—and you’ve got a total of nine.Based on the responses, every port on a bridge or switch is a separate collision domain.

[lammle]

In figure 1.4 there are three broadcast domains and nine collision domains, my books spells this out perfectly and explains why there is that many of each.

Every port on a bridge or switch is a collision domain so there are nine interfaces in use in that figure, and the router has three interfaces in use, so three broadcast domains.

My book is not wrong, nor is there an error (yes, I am looking at both my 6th and 7th edition books.
Todd Lammle

[Michael950]

Hi, I started reading the 5th edtion deluxe-version, and I also have a question/comment to ch1.

Figure 1.4 the text bellow says "Also, the bridge only created two collision domains.." Isnt it 3 instead? Later the calculations are correct and used 3.

I like really like the style of the book so far, it almost feels as I am listening to a speaker in the classroom.

Thanks

Laszlo

I was confused by the same text on page 9.
I am not debating the count of 9 collision domains. Just the confusion caused by the text I quoted directly from the book.

I do not appear to be alone in the confusion.

*****
Also, this appears to be a popular thread based on the number of views. Currently, this thread is number 4 for this forum.

I was hoping my post will help someone in the future.

[lammle]

yes, if it said "the bridge created two collision domains", then that is incorrect. I obviously fixed it since I can't find the error, even in the 6th edition (fixed in a later print release).
I hope this resolves this issue.
Thanks!
Todd Lammle

[whiteseat]

Sorry Mr Todd Lammle,

I have the same confusion in the Figure 1.4.


I too had trouble figuring this one out. What threw me off was I assumed those were hubs attached to the hosts on the left side of the diagram. When I realized they were switches, it became clear. I have attached a pic with what I think are the collision domains cirlced. Can someone please verify that these are the 9 collision domains?