Collision Domains & Broadcast Domains

Mar 09, 2005 5 Replies
R

I seem to keep screwing up practice questions that have network diagrams and you have to identify the number of collision and broadcast domains. Is there an easy way to tackle this type of question?



I've tried using this approach:



Count each interface on the router as 1 collision domain and 1 broadcast domain.



Count each connected interface (including interfaces between switches and routers) on a switch as a collision domain and unless VLANS are used, count each switch as a broadcast domain. If VLANS are used count



1 broadcast domain per VLAN.

Count 1 collision domain and 1 broadcast domain for each hub



Is this the correct logic? Like I said, I always seem to get the number of collision or broadcast domains wrong. I suspect becuase the way the design is interconnected; hence my question on a way to consistently get the right answer.



Thanks,



RB


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K

A router can break collision domains and broadcast domains. Therefore, count each interface of a router as 1 collision domain and

1 broadcast domain.

A switch or bridge can only break collision domains, but not broadcast domains unless VLANs are used. Therefore, count each interface of a switch as 1 collision domain, and each VLAN as 1 broadcast domain.

A hub or repeater cannot break collision domains and broadcast domains. Therefore, no need to consider hubs and repeaters when counting collision domains or broadcast domains.

KPLAB

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M

just don't forget that connected elements should only be counted once. For example, a 10 port switch connected to a 1 port router only has 10 collision domains; two routers connected to three networks (only one in common) only have three broadcast domains.

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R

I'm with you on the first 2 points. However don't you still have to count a hub as one collision domain and one broadcast domain if it happens to be sitting on the other side of a router?

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K

You only need to count the router interface. For example, if the router interface is connected to a hub which connects 5 hosts, all the

5 hosts are sharing the same collision domain and broadcast domain. The number of collision domain and broadcast domain remain the same if the router interface is connected to a host directly rather than connected to the hub.

KPLAB

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