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A broadcast domain is a grouping of
collision domains that are connected by Layer 2 devices.
 Breaking up a
LAN into multiple collision domains increases the opportunity for each
host in the network to gain access to the media. This effectively
reduces the chance of collisions and increases available bandwidth for
every host. But broadcasts are forwarded by Layer 2 devices and if
excessive, can reduce the efficiency of the entire LAN. Broadcasts
have to be controlled at Layer 3, as Layer 2 and Layer 1 devices have
no way of controlling them. The total size of a broadcast domain can
be identified by looking at all of the collision domains that the same
broadcast frame is processed by. In other words, all the nodes that
are a part of that network segment bounded by a layer three device.
Broadcast domains are controlled at Layer 3 because routers do not
forward broadcasts. Routers
actually work at Layers 1, 2, and 3. They, like all Layer 1 devices,
have a physical connection to, and transmit data onto, the media. They
have a Layer 2 encapsulation on all interfaces and perform just like
any other Layer 2 device. It is Layer 3 that
allows the router to segment broadcast domains.
In order for a packet to be forwarded
through a router it must have already been processed by a Layer 2
device and the frame information stripped off. Layer 3 forwarding is
based on the destination IP address and not the MAC address. For a
packet to be forwarded it must contain an IP address that is outside
of the range of addresses assigned to the LAN and the router must have
a destination to send the specific packet to in its routing table.
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