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The two major problems
with traditional networks have always been availability and performance.
These two problems are both impacted by the amount of bandwidth
available. In a single collision domain, frames are visible to all
devices on the LAN and are free to collide.
Multiport Layer 2 devices (e.g., bridges
and switches) are used to segment the
LAN into discrete collision domains and forward Layer 2 data frames to
only the segment of the network that contains the destination address.
Because the Layer 2 ports separate the LAN into distinct physical segments,
they also help to resolve issues related to the distance limitations
of Ethernet.
However, frames containing the broadcast
Media Access Control (MAC) address are still flooded throughout the
entire network as shown in Figure .
A single network device could malfunction and flood the network with
"noise" and this could bring down the network. This is where
routers come in. Since routers operate at Layer 3 of the Open System
Interconnection (OSI) model, they are capable of making intelligent
decisions regarding the flow of data to and from a network segment.
Traffic that can affect network
performance is traffic that polls the network about component status or
availability and advertises network component status or availability.
Two common types of broadcasts that poll the network are IP Address
Resolution Protocol (ARP) requests as shown in Figure
and NetBIOS name requests. These broadcasts are normally propagated
across an entire subnet and expect the target device to respond directly
to the broadcast.
In addition to broadcasts, multicast
traffic can also consume a large amount of bandwidth. Multicast traffic
is propagated to a specific group of users. Depending on the number of
users in a multicast group or the type of application data contained in
the multicast packet, this type of broadcast can consume most, if not
all, of the network resources. An example of a multicast implementation
is the Cisco IP/TV solution, which uses multicast packets to transport
multimedia such as audio and video.
As networks grow, so does the amount of
broadcast traffic on the network. Excessive broadcasts reduce the
bandwidth available to the end users and force end-user nodes to waste
CPU cycles on unnecessary processes. In a worst-case scenario, broadcast
storms can effectively shut down the network by monopolizing the
available bandwidth.
Two methods can address the broadcast
issue for large switched LAN sites. The first option is to use routers
to create many subnets and logically segment the traffic as shown in
Figure .
Broadcasts do not pass through routers. One problem with this method is
that, although this approach will contain broadcast traffic, the CPU of
a traditional router will have to process each packet. This scenario can create a bottleneck in the network.
A second option would be to implement virtual
LANs (VLANs) within the switched network as shown in Figure .
For the purpose of this curriculum, VLANs are basically defined as
broadcast domains. A VLAN is a group of end devices that populate
multiple physical LAN segments and switch ports; they communicate as if
they were on a single LAN segment. One of the primary
benefits of VLANs is that LAN switches (by creating VLANs) can be used
to effectively contain broadcast traffic and manage traffic flows.
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