Broadcast Control

Broadcast Control
Broadcasts occur in every protocol, but how often they occur depends on the protocol, the
application(s) running on the network, and how these services are used. VLANs can define
smaller broadcast domains, which means that it is possible to stop application broadcasts to
segments that do not use the application.
Although some older applications have been rewritten to reduce their bandwidth needs,
there is a new generation of applications that are bandwidth greedy, consuming all they can
find. These are multimedia applications that use broadcasts and multicasts extensively. Faulty
equipment, inadequate segmentation, and poorly designed firewalls can also add to the problems
of broadcast-intensive applications.
For the moment, you should consider multicast traffic to be the same as broadcast
traffic. The switch has no default knowledge of multicast groups, and forwards
it out of every port. We deal with this issue in detail in Chapter 18,
“Multilayer Switching.”
These bandwidth-gobbling applications have added a new factor to network design because
broadcasts can propagate through the switched network. Routers, by default, send broadcasts
only within the originating network, but layer 2 switches forward broadcasts to all segments.
This is called a
flat network
because it is one broadcast domain.
As an administrator, you must make sure the network is properly segmented to keep problems
on one segment from propagating through the internetwork. The most effective way of doing this
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• Each segment has its own collision domain.
• All segments are in the same broadcast domain.
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Chapter 14 
VLANs, Trunks, and VTP
is through switching and routing. Because switches have become more cost-effective, a lot of companies
are replacing the hub-and-router network with a pure switched network and VLANs. The
largest benefit gained from switches with defined VLANs is that all devices in a VLAN are members
of the same broadcast domain and receive all broadcasts. The broadcasts, by default, are filtered
from all ports that are on a switch and are not members of the same VLAN.
Every time a VLAN is created, a new broadcast domain is created. VLANs are used to stop
broadcasts from propagating through the entire internetwork. Some sort of internal route processor
or an external router must be used in conjunction with switches to provide connections
between networks (VLANs).