designed the most efficient path to the destination.
Single points of failure need to be analyzed, as well. As we stated earlier, every large−network user has
suffered through his or her share of network outages and downtime. By analyzing all the possible points of
failure, you can implement redundancy in the network and avoid many network outages. Redundancy is the
addition of an alternate path through the network. In the event of a network failure, the alternate paths can be
used to continue forwarding data throughout the network.
The last principle that you should consider when designing your network is the behavior of the different
protocols. The actual switching point for data does not have to be the physical wire level. Your data can be
rerouted at the Data Link and Network layers, as well. Some protocols introduce more network traffic than
others. Those operating at Layer 2 can be encapsulated or tagged to create a Layer−3−like environment. This
environment allows the implementation of switching, and thereby provides security, protocol priority, and
Quality of Service (QoS) features through the use of Application−Specific Integrated Circuits (ASICs) instead
of the CPU on the switch. ASICs are much faster than CPUs. ASICs are silicon chips that provide only one or
two specific tasks faster than a CPU. Because they process data in silicon and are assigned to a certain task,
less processing time is needed, and data is forwarded with less latency and more efficiency to the end
destinations.
In order to understand how switches work, we need to understand how collision domains and broadcast
domains differ.
Collision Domains
A switch can be considered a high−speed multiport bridge that allows almost maximum wire−speed transfers.
Dividing the local geographical network into smaller segments reduces the number of interfaces in each
segment. Doing so will increase the amount of bandwidth available to all the interfaces. Each smaller segment
is considered a collision domain.
In the case of switching, each port on the switch is its own collision domain. The most optimal switching
configuration places only one interface on each port of a switch, making the collision domain two nodes: the
switch port interface and the interface of the end machine.
Let’s look at a small collision domain consisting of two PCs and a server, shown in Figure 1.4. Notice that if
both PCs in the network transmit data at the same time, the data will collide in the network because all three
computers are in their own collision domain. If each PC and server was on its own port on the switch, each
would be in its own collision domain.
Figure 1.4: A small collision domain consisting of two PCs sending data simultaneously to a server.
Switch ports are assigned to virtual LANs (VLANs) to segment the network into smaller broadcast domains.
If you are using a node attached to a switch port assigned to a VLAN, broadcasts will only be received from
members of your assigned VLAN. When the switch is set up and each port is assigned to a VLAN, a
broadcast sent in VLAN 1 is seen by those ports assigned to VLAN 1 even if they are on other switches
attached by trunk links. A switch port can be a member of only one VLAN and requires a Layer 3 device such
as an internal route processor or router to route data from one VLAN to another.
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