Introduction to LAN switching
Welcome to the world
of LANS, in the previous couple of chapters, we learnt about routers and how
they are used in the networking environments. In the next couple of chapters,
we will learn about LANs and the main device we will use will be the SWITCH. The
concepts we will learn will also tie in with routing concepts.
LAN
DESIGN
Overview
In today’s business
environment, businesses need information to survive. With technology, this has
been made possible, the use of new methods of communicating such as the use of
voice, video, data which is transmitted over networks is crucial. As such, we
need to design LANs with these needs in mind. In this chapter, we will discuss
some of the considerations to make while we design the LAN. We will look at the
hierarchical LAN model and its benefits, some design considerations as well as
the benefits of well-designed LANs. This chapter is meant to introduce you to
the world of LANs.
LAN0 design concepts
CISCO not only designs
and produces network equipment, but also they focus on developing the most
optimal way to use their devices, as such when designing a LAN network, they
recommend that a hierarchical model. In this type of architecture, there are a
few things that have to be observed:
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Network segmentation
and broadcast traffic management – this is mainly through the use of VLANs
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Security
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Easy configuration and
management of the switches
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Redundancy
These concepts will be
explored in more detail as we explore LAN design.
Hierarchical layered
model in LAN design
As mentioned earlier,
the design of a LAN network is critical to communication within the enterprise,
when using the hierarchical model as recommended by CISCO, there are three
layers that we should implement depending on the size of the organization.
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Core layer
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Distribution layer
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Access layer.
The figure below shows
how the implementation of this hierarchy can be achieved.
Starting from the
bottom, we have the access layer. This is the layer that connects to end user
devices such as PCs, printers, IP phones among others.
The distribution
layer, is meant to aggregate the data from the access layer. This layer
controls the traffic in the lower levels and prioritizes traffic based on
organizational policies that have been implemented during configuration of the
switches. Typically, this level should be redundant and made up of faster
switches than the access layer.
The core layer, is
responsible for high-speed switching in the network. Typically, this layer
should consist of the fastest switches in the network and offer the highest
bandwidth since communication to other networks from the lower levels is
forwarded through these switches.
Benefits of a
hierarchical model
§
Scalability – when you
implement a network a hierarchical network model, expansion is simplified since
all the roles are well defined. For example, if you have 5 access layer
switches, connected to 2 distribution layer switches, you can add the access
layer switches until all the ports on the distribution switches are filled up.
§
Redundancy – this is
achieved when the switches in each layer are connected to two or more devices
at another level. If one device at the higher level in the hierarchy fails, the
lower level switch automatically fails over to the other switch. Redundancy is
achieved at the distribution and core layers.
§
Performance – it is
recommended that core layer switches should have very fast switching abilities.
The distribution switches should also be very fast and redundant. The result of
using very fast core and distribution layer switches would guarantee very fast
networks.
§
Security – the
security of the network is enhanced since at each layer of the model, there are
several security measures that can be put in place; for example switch ports at
the access layer can be configured with port security, segmentation of the
distribution layer using VLANs is also another security feature.
§
Manageability is the
ability to make configuration changes in the network, the use of the
hierarchical model eases management of the switches. For example, making
changes on one layer would be simplified since we can assume that the role of
switches in that layer all perform similar functions, further, the modular
design means that management does not mean that the network is down due to
maintenance due to redundancy.
Considerations when
choosing a switch
§
When deciding the
switch we should implement for our LANs, there are several considerations that
we need to take in mind. These might be influenced by the organizational
policies while others might be influenced by the technological needs.
§
Switches with fixed
configurations are switches that cannot be modified by adding additional
modules, these are lower level switches and are ideal for the access layer
functions.
§
For more flexibility,
we might need modular switches, these switches typically allow us to install
modules such as more switching ports, these would be ideal for rapidly
expanding networks that need to be changed frequently.
§
To provide high
bandwidth, we may need to interconnect special types of switches which have a
stackable ability using a backplane cable. These would be ideal for high
bandwidth requirements in a large network at the core layer.
§
Port density – this is
the number of ports on a switch. In many cases you will find switches with 24
or 48 port switches. This can be a design consideration since you may need to
consider the inter-switch connections.
§
Forwarding rates are
the processing capabilities of the switch. The forwarding rate is measured by
calculating how much data the switch can process in a second. This is different
from the bandwidth that is available on its ports.
§
In most modern
networks, the use of IP phones is prevalent, most of these devices get power
over the LAN interfaces connected to switches using a technology called POE
(Power over Ethernet). As such, when deciding which devices to buy, PoE should
be a feature that should not be overlooked.
§
In recent times,
switch designs have been changed so as to support layer 3 functionality, as you
may already know, switches work at layer 2 of the OSI model, however,
implementing layer 3 switches gives more options such as routing, IP addressing
and other options.
Access layer switch
features
There are several
features that a switch at each level of the hierarchical model should have. As
we mentioned earlier, the access layer is the lowest level in the hierarchical
LAN architecture, at this level user devices gain access to the network over a
number of devices. As such, the features at this level include: VLAN support on
the switches, Fast Ethernet and Gigabit Ethernet links, PoE and support for
link aggregation so as to increase the switching speed.
Security is important
in our networks, at this layer, we can implemnent several security measures
such as port security to control access to the network.
CISCO recommends that
VLANs be localized to a switch, the switches at this level should have support
for VLANs for a variety of purposes.
Link aggregation is
the ability to use multiple links at the same time. This is a more effective
way to use the bandwidth available on the switches.
To support multiple
devices on a single port, PoE is an important feature, it allows us to use the
switch to power certain devices in our network such as IP phones and Wireless
controllers.
The ports on access
layer switches should be fast enough to support the evolving bandwidth needs of
the enterprise. As such, Fast Ethernet which offer speeds of up to 100Mbps and
Gigabit Ethernet links which offer speeds of up to 1Gbps should be used.
Distribution layer
features
At the distribution
layer, communication across the various access layer switches should be
supported, this means that these switches should offer more features than the
access layer switches. Features such as redundancy, faster ports than the
access layer, layer 3 support should be implemented at this layer.
§
The use of security
policies is a security feature that should be implemented at the distribution
layer, some of these may include the use of access lists.
§
Inter-vlan routing
which is making communication between different VLANs possible should be
available at this layer.
§
The ports at this
layer should be very fast, typically, Gigabit Ethernet and 10 gigabit Ethernet
links should be used. These ports should be aggregated and redundancy should be
implemented between the switches.
§
At this layer, we need
to prioritize the traffic from our access layer, as such, QOS (Quality of
Service) mechanisms should be implemented.
NOTE: at the distribution layer, the use of layer 3 capable
switches is highly recommended so as to support most of the features mentioned
above.
Core layer features
The core layer of the
network is the main link between our internetwork and other networks such as
external networks. At this layer of the hierarchical model, there should be
very fast switching, security policies, redundancy, layer 3 functionality and quality
of service. In some organizations, the core layer may not be needed if the
network is small.
§
At the core layer, we
should have very fast switches, typically operating at 10 gigabit speeds and
above. This is to support the requirements of all the access and distribution
layer switches.
§
At this level, the use
of security policies to control access should be implemented. This means that
the switches at this layer should have layer 3 support.
§
The core layer is
sometimes implemented as the gateway to external networks and therefore
redundancy is also an important element.
In the forthcoming
chapters, we will discuss some of these concepts in detail through the networks
that we will design and implement. The concepts in this chapter are meant to
give you a firm foundation on the LAN architecture as recommended by CISCO. For
more on this, you should conduct more research to discover best practices when
it comes to designing and implementing LAN networks.
Summary
In this chapter, we
have introduced the LAN. We have looked at the hierarchical layered model when
designing LANs. In the next chapter, we will discuss switch concepts and the
basic configuration of a switch.
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