Content Overview Designing a network
can be a challenging task that involves more than just
connecting computers together. A network requires many features
in order to be reliable, manageable, and scalable. To design
reliable, manageable, and scalable networks, a network designer
must realize that each of the major components of a network has
distinct design requirements. Network design is becoming more
difficult despite improvements in equipment performance and
media capabilities. Using multiple media types and
interconnecting LANs with other external networks makes the
networking environment complex. Good network design will
improve performance and also reduce the difficulties associated
with network growth and evolution. A LAN spans a single room, a
building, or a set of buildings that are close together. A
group of buildings that are on a site and belong to a single
organization are referred to as a campus. The design of larger
LANs includes identifying the following: - An access
layer that connects end users into the LAN
- A
distribution layer that provides policy-based connectivity
between end-user LANs
- A core layer that provides the
fastest connection between the distribution points
Each of these LAN design layers requires switches that are best
suited for specific tasks. The features, functions, and
technical specifications for each switch vary depending on the
LAN design layer for which the switch is intended.
Understanding the role of each layer and then choosing the
switches best suited for that layer ensures the best network
performance for LAN users. Students completing this module
should be able to: - Describe the four major goals of
LAN design
- List the key considerations in LAN
design
- Understand the steps in systematic LAN
design
- Understand the design issues associated with
the Layer 1, 2, and 3 LAN structure, or topology
- Describe the three-layer design model
- Identify the
functions of each layer of the three-layer model
- List
Cisco access layer switches and their features
- List
Cisco distribution layer switches and their features
- List Cisco core layer switches and their features
Content 5.1 LAN Design
5.1.1 LAN design goals The first step in designing a LAN is
to establish and document the goals of the design. These goals
are unique to each organization or situation. The following
requirements are usually seen in most network designs:
- Functionality – The network must work. The network
must allow users to meet their job requirements. The network
must provide user-to-user and user-to-application connectivity
with reasonable speed and reliability.
- Scalability – The network must be able to grow. The
initial design should grow without any major changes to the
overall design.
- Adaptability – The network
must be designed with a vision toward future technologies. The
network should include no element that would limit
implementation of new technologies as they become available.
- Manageability – The network should be designed
to facilitate network monitoring and management to ensure
ongoing stability of operation.
Interactive
Media Activity Matching: LAN Design Goals Matching When the
student has completed this activity, the student will be able
to understand the terms, definitions, and goals in network LAN
design.
Content 5.1 LAN Design
5.1.2 LAN design considerations Many organizations have
been upgrading existing LANs or planning, designing, and
implementing new LANs. This expansion in LAN design is due to
the development of high-speed technologies such as Asynchronous
Transfer Mode (ATM). This expansion is also due to complex LAN
architectures that use LAN switching and virtual LANs
(VLANs).To maximize available LAN bandwidth and performance,
the following LAN design considerations must be addressed:
- The function and placement of servers
- Collision detection issues
- Segmentation
issues
- Broadcast domain issues
Servers
provide file sharing, printing, communication, and application
services. Servers typically do not function as workstations.
Servers run specialized operating systems, such as NetWare,
Windows NT, UNIX, and Linux. Each server is usually dedicated
to one function, such as e-mail or file sharing. Servers can be
categorized into two distinct classes: enterprise servers and
workgroup servers. An enterprise server supports all the users
on the network by offering services, such as e-mail or Domain
Name System (DNS). E-mail or DNS is a service that everyone in
an organization would need because it is a centralized
function. However, a workgroup server supports a specific set
of users, offering services such as word processing and file
sharing. Enterprise servers should be placed in the main
distribution facility (MDF). Traffic to the enterprise servers
travels only to the MDF and is not transmitted across other
networks. The reviewer's rewrite leaves out the important point
about the traffic to the enterprise servers traveling only to
the MDF. Ideally, workgroup servers should be placed in the
intermediate distribution facilities (IDFs) closest to the
users accessing the applications on these servers. By placing
workgroup servers close to the users, traffic only has to
travel the network infrastructure to an IDF, and does not
affect other users on that network segment. Layer 2 LAN
switches located in the MDF and IDFs should have 100 Mbps or
more allocated to these servers. Ethernet nodes use CSMA/CD.
Each node must contend with all other nodes to access the
shared medium, or collision domain. If two nodes transmit at
the same time, a collision occurs. When this occurs, the
transmitted frame is destroyed, and a jam signal is sent to all
nodes on the segment. The transmitting nodes wait a random
period of time, and then resend the data. Excessive collisions
can reduce the available bandwidth of a network segment to 35%
or 40% of the bandwidth available. Segmentation is the process
of splitting a single collision domain into smaller collision
domains. Creating smaller collision domains reduces the number
of collisions on a LAN segment, and allows for greater
utilization of bandwidth. Layer 2 devices such as bridges and
switches can be used to segment a LAN into smaller collision
domains. Routers can achieve this at Layer 3. A broadcast
occurs when the destination media access control (MAC) data
frame address is set to FF-FF-FF-FF-FF-FF. A broadcast domain
refers to the set of devices that receive a broadcast data
frame originating from any device within that set. All hosts
that receive a broadcast data frame must process it. Processing
the broadcast data will consume the resources and available
bandwidth of the host. Layer 2 devices such as bridges and
switches reduce the size of a collision domain. These devices
do not reduce the size of the broadcast domain. Routers reduce
the size of the collision domain and the size of the broadcast
domain at Layer 3.
Content 5.1 LAN
Design 5.1.3 LAN design methodology For a LAN to
be effective and serve the needs of its users, it should be
designed and implemented according to a planned series of
systematic steps. These steps include the following: -
Gather requirements and expectations
- Analyze
requirements and data
- Design the Layer 1, 2, and 3
LAN structure, or topology
- Document the logical and
physical network implementation
The information
gathering process helps clarify and identify any current
network problems. This information includes the organization's
history and current status, their projected growth, operating
policies and management procedures, office systems and
procedures, and the viewpoints of the people who will be using
the LAN. The following questions should be asked when gathering
information: - Who are the people who will be using the