Monday, 5 October 2009

Collision Domains vs Broadcast Domains



Collision Domains

layer 1 of the OSI model

A hub is an entire collision domain since it forwards every bit it receives from one interface on every other interfaces

A bridge is a two interfaces device that creates 2 collision domains, since it forwards the traffic it receives from one interface only to the interface where the destination layer 2 device (based on his mac address) is connected to.

A bridge is considered as an "intelligent hub" since it reads the destination mac address in order to forward the traffic only to the interface where it is connected

A switch is a multi-interface hub, every interface on a switch is a collision domain. A 24 interfaces switch creates 24 collision domains (assuming every interface is connected to something, VLAN don't have any importance here since VLANs are a layer 2 concept, not layer 1 like collision domains)

Broadcast Domains

Layer 2 of the OSI model

A switch creates an entire broadcast domain (provided that there's only one VLAN) since broadcasts are a layer 2 concept (mac address related) routers don't forward layer 2 broadcasts, hence they separate broadcast domains

With all this information, you can say that on your diagram, there are 2 broadcast domains (1 router that separates 2 LAN segments composed by one or many switches, with only 1 VLAN per segment).

There are 8 collision domains, one per pair of devices connected to each other (switch to router, switch to switch, switch to computer etc...) since we are talking about layer 1 concept (physical connection).

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