colission domain

[cmurillo]

hi Im new in this forum so I need your help about a cuestion, in page 9 from ccna study guide 640-802 the figure show (in other things) a router conected to a bridge and this one to two hubs.
In tis case the books say "the bridge only created two collision domains, so each device connected to a hub is in the same collision domain as every other devide connected to that same hub"

I thing is correct but, the conecction between router and bridge is not a collision domain too? then there are tree collision domain

What is the correct answer? Lammle say that the correct answer is the cisco answer then, does cisco say the correct answer is two collision domain?

can anybody helpme?

thank you very much

[dbeare]

I'm not exactly sure what you're asking, but lets look at the diagram on page 9. At the center of the diagram is a router with three interface connections, one to a switch, one to a bridge, one to a router. Since routers break up broadcast domains (and collision domains), we know that this diagram contains 3 broadcast domains. Lets look at each of the interface connections 1 at a time.

Interface 1: Bridge
The network bridge has 2 hubs connected to it. Remember how hubs work, they are simply repeaters that send out all packets to all connected ports. Bridges, like switches, create new collision domains. Each interface on the switch is a collision domain. Since there are two ports on the switch hooked to two switches, the bridge contains 2 collision domains.

Interface 2: Switch
A switch, like a bridge, breaks up collision domains. There are three switches on this side, one connected to the router and two that are connected to that switch. Each connection on each switch is essentially its own collision domain. So, the connection from the switch to the router is a collision domain, the connection from the central switch to the other two switches is two more collision domains, and the connection from each of those switches to a host are two more collision domains. So, in total, there are 5 collision domains on this network segment.

Interface 3: Hub
This is an easy one, because hubs do not break up collision domains, nor do they break up broadcast domains, they simply repeat the electrical signal out all of their ports. This means that this interface is 1 broadcast domain and 1 collision domain.

Hope this helps!

[BlackJackiv2008]

Hi Mr dbeare,

From your calculation, there are only 8 collision domains, but, if we read the answer by Mr Todd Lammle on Pg10, there is 9 collision domains.
Where should the 9th collision domain be?

From the Glossary Definition at the back of CCNA Study Guide,6th edition:

Collision Domain: The network area in Ethernet over which Frames that have collided will be detected(Hub, Repeater has collisions). From a definition above, and looking back to Figure 1.5, PC connected directly to the switch port (without a hub) is also considered as a collision domain? Why?

Your explanation is deeply appreciated. :)

[kesnet]

page 11.

be:
each port on the switch is a separate CD and...:eek:

[pm123]

with resoect to the router, every port is a new broadcast domain as router breaks the broadcast domains by default. and automatically collision domains get broken, but this with respect to the router. with respect to the hub, all connections constitute the only collision domain, so there is no question of breaking the broadcast domain w.r.t hub. a swtich on the other hand, would have each port as a separate collision domain but w.r.t switch there is only one broadcast domain.
parshwa

hi Im new in this forum so I need your help about a cuestion, in page 9 from ccna study guide 640-802 the figure show (in other things) a router conected to a bridge and this one to two hubs.
In tis case the books say "the bridge only created two collision domains, so each device connected to a hub is in the same collision domain as every other devide connected to that same hub"

I thing is correct but, the conecction between router and bridge is not a collision domain too? then there are tree collision domain

What is the correct answer? Lammle say that the correct answer is the cisco answer then, does cisco say the correct answer is two collision domain?

can anybody helpme?

thank you very much

[kellycpp]

I believe that cmurillo is correct.

The connection from the bridge to the router is actually a collision domain (the 9th) from the perspective of the bridge as well as from the perspective of the router. I am thinking that the sentence associated with the idea of the two CDs in the book is referring to the two hubs being the paths of 2 of the CDs. But this sentence ignored the third CD on the third port of the bridge. Each bridge port has a separate CD (even if it is not connected to any actual non-network device like a PC). As well, each port of a router establishes a separate CD. Therefore the connection via bridge is 3 + connection to lower hubs is 1 + connection to switches to the left has 5 for a total of 9.

Most of my time spent writing is actually spent in editing for the tweaks which provide clarity. This editing can produce better phrasing for better understanding of the whole issue; easily 3/4 editing and 1/4th initial writing. I have made half a dozen edits on this small offering! Sometimes you have to read from this perspective also? ;)