LAN Design
Semester 3, Chapter 4
Allan Johnson
Table of Contents
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Design Goals & Components
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Network Design Methodology
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Layer 1 Design
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Layer 2 Design
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Layer 3 Design
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Design Goals &
Components
Table of Contents
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LAN Design Goals
Critical to design is insuring a fast and
stable network that will scale well as
the organization grows
Design steps are...
1. Gather & establish design goals based on user
requirements
2. Determine data traffic patterns now & in the future
3. Define Layer 1, 2, & 3 devices & the LAN/WAN
topologies
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4. Document physical & logical network implementation
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Establish the Design Goals
Although organizations are unique to the
customer, the following requirements
tend to be generic to all. The network
must have...
 Functionality--speed and reliability
 Scalability--ability to grow without major changes
 Adaptability--easily implements new technologies
 Manageability--facilitates monitoring and ease of
management
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Critical Components of LAN Design
With the emergence of high-speed
technologies and complex LAN
technologies, the following critical
components need addressing in design
 Function & placement of Servers
 Collision Detection
 Microsegmentation
 Bandwidth v. Broadcast domains
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Placement of Servers
Servers now perform special functions
and can be categorized as either...
 Enterprise Servers--supports all users on the network
DNS and mail servers
should be placed in the MDF
or...

Workgroup Servers--supports a specific set of users
file serving such as specialized databases
should be place in the IDF closest to users
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Graphic
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Intranets & Collisions
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Intranets are internal to the organization and are not accessible by
the public over the Internet.
Intranet Servers use browsers to provide access to authorized users.
This has caused an increase in needed bandwidth. Therefore,
design must address...
 Type of data to be accessed
 Server privileges
 Outfitting desktops with faster connectivity
 More processing power
 10/100Mbps NICs to provide migration to switched technologies
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Collision detection and minimization has become a major concern as
users attempt to access the same server.
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As we’ve seen, switches can provide dedicated bandwidth to
minimize or eliminate collsions.
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Broadcasts & Segmentation
Layer 2 devices
segment collision
domains
Layer 3 devices
segment
broadcast
domains
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Bandwidth v. Broadcast Domains

A bandwidth domain is shared
by all devices on a single
switched port.
Synonymous with collision
domain

A broadcast domain is shared
by all devices on a single
router interface.
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Network Design
Methodology
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Gathering & Analyzing Requirements

Gathering data
about the
organization
includes:
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Network Availability
Network design seeks to
provide the greatest
availability for the least cost.
 Factors that affect availability
include...

Throughput
Response time
Access to resources

In the graphic, what type of
server is each and where
should each be placed?
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Physical Topologies
In the CCNA curriculum, we
concentrate on the
star/extended star physical
topology which typically
uses the Ethernet 802.3
standard.
 Why? Because it is the
most popular topology used
in LANs.
 The next three sections,
evaluate the extended star
by layers.
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Layer 1 Design
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Ethernet Cable Runs
The physical cabling (also called the cable plant) is the most
important Layer 1 issue to consider when designing a
network.
 Design issues include...

Type of cable to use (twisted-pair, coax, fiber)
Where to use each type (e.g. fiber on the backbone)
How far each run must travel before being terminated (twisted-pair is
limited to what distance?)

In an existing LAN, a cable audit is performed to determine
where upgrading and/or replacement of bad cables is
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needed.
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MDF & Other 568A Acronyms

Whether the LAN is a star
or extended star, the MDF is
the center of the star.
From the workstation to the
telecommunications outlet,
the patch cable should be no
more than 3m.
From there to the patch
panel, called the HCC, no
more than 90m.
From the patch panel (the
HCC) to the switch, no more
than 6m.
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MDF & Other 568A Acronyms
When distances to the MDF
are more than 100m, an IDF is
normally added.
 The cable run from the IDF to
the MDF is called the VCC
and is usually fiber.
 VCC is just another name for
the backbone.
 By adding more wiring closets
(more IDFs), you create
multiple catchment areas
(Click of graphic button)
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Graphic
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10BaseT and 100BaseT Ethernet
100 BaseT (also called Fast Ethernet) is
now the standard for connecting IDFs to
the MDF.
 Although you can run Fast Ethernet over 10BaseT cabling
(twisted pair), the distance limitation means fiber is most
often used
 The 100BaseT standard running on twisted paid is called
100BaseTX
 On fiber, it is called what?
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 What is Gigabit Ethernet called?
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Layer 1 Logical Documentation

Layer 1 logical documentation
is concerned with...
 exact location of MDF/IDF
 type & quantity of cabling
 room locations & # of drops
 port numbers
 cable labels
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Notice Layer 1’s logical
documentation shows nothing
about logical addressing
The Logical Diagram and Cut
Sheet are primary tools for
design, but are crucial to the
tech who is troubleshooting.
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Layer 2 Design
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Common Layer 2 Devices

The two most common
Layer 2 devices are...
Bridges and
LAN Switches
Both provide the added
benefit of what?
 Segmenting collision
domains into microsegments.
 Switches can also provide connections of unlike
bandwidth (e.g., 100Mbps to the server & 10Mbps to
workstations). This is called...?
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Sizing Collision Domains
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In a switched LAN environment
using hubs, the bandwidth of each
switched port is shared by all the
devices. Therefore, they also share
the same collision domain.
To determine the bandwidth per
host, simply divide the port’s
bandwidth by the number of hosts
(see graphic).
In a pure switched LAN environment
where each host has its own port,
the size of the collision domain is 2.
If running full-duplex, then the
collision domain is eliminated. Why?
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Migrating to 100BaseT

As long as your workstations all
have 10/100 NICs, increasing the
bandwidth is easy.
Replace the hub with a 100Mbps
capable hub and patch the HCC
into a 100Mbps port on the switch.
In addition, you can add another
100Mbps VCC from the IDF to the
MDF, which provide 200 Mbps to
the IDF’s switch.
In the graphic, the red lines
represent migrating to 100Mbps.
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Layer 3 Design
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Routers and Design
Routers provide both
physical and logical
segmentation.
 Physically, routers segment
what?
 Logically, routers segment
according to Layer 3
addressing dividing the LAN
into logical segments called
subnets.
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VLANs & Broadcast Domains
As we learned in Chapter
3, VLAN capable switches
help routers contain
broadcasts.
 The graphic shows two
broadcast domains.
 Notice there are also two
subnets. How do we know
that?
 The router provides
communication between
the two VLANs.
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Diagramming a LAN with Routers
Notice in the graphic that
the two networks are kept
separate by the router.
 Each switch serves a
different network regardless
of the physical location of
the devices.
 To create another physical
network in a structured
Layer 1 wiring scheme,
simply patch the HCC and
VCC into the correct switch.
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Logical & Physical Network Maps

After determining your Layer 1, 2, and 3 design, you can
create your addressing (logical) and physical maps. These
are invaluable. They
Give a snapshot of the network
Show subnet mask info
Help in troubleshooting
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