Vanessa Avalos
Cisco Networking Academy Semester 3
Lecture Notes

Chapter 4
LAN Design

The first step in designing a LAN is to establish and document the goals of the design.
 There are different goals within each organization.  Below are some things that tend to show up in most LAN designs.

• Functionality – The ability to do what it needs to do at a high speed and do it all the time.
• Scalability – The ability to grow without any major changes in the design
• Adaptability – Designed so that no element will limit the addition of new technology.
• Manageability – Designed to ensure stable operation and ease of network monitoring and management.

 Following are some critical components of LAN design that should be addressed by network designers:

Function and placement of servers:

•  Servers provide, file sharing, printing, communications, and application services
• Each server is dedicated to one function, for example, an email server
• Servers have specialized operating systems, win 2K, Unix and Linux
• Servers can be put into two categories:
• Enterprise servers – All users, put in MDF
• Workgroup servers – specific set of users, put in IDFs
•  Servers need lots of bandwidth. It’s a good idea to allocate 100Mbps or more.
• Collisions:  Segment LAN to limit collisions.  When you segment the LAN, take future growth into consideration.  Keep your collision domains small.

Broadcast vs. Bandwidth

Remember, segmenting limits collisions, not broadcast.
Bandwidth domains depend on the total amount of traffic.
Scalability for a broadcast domain depends on the total amount of traffic.

Steps for designing a LAN

1. Gathering user requirements and expectations.  This includes: 
       Information on the corporate structure:

• History
• Current Status
• Projected Growth
• Management Procedures
• Office systems and procedures
• View points of people using the LAN

2. Business information flow:

• What is mission critical
• What are the key operations to the business
• What protocols are already in use on the network
• What hosts are supported

3. What applications are in use and who uses them

4. Current topology

• Who’s in charge of addressing, naming, configuration and topology design.
• What is the current topology 
• What are the current business resources, software/hardware

5. Performance of current LAN topology

• Latency
• Bottlenecks
• What is the users opinion about current LAN

Step 1 (#3):
Analyze the requirements – traffic intense software
Factors that affect availability:

• Throughput
• Response time
• Access to resources

Network design is looking for the greatest availability at the least cost.

Step 2:
Analyze the data that has been gathered:

• User requirements
• Steps must be taken to ensure that the information requirements of the organization and all it’s users are met.

Step 3:
Decide on LAN topology (the first 3 layers)

Step 4:
Develop the Layer 1 LAN topology
    Wires/cables

• Type of media (CAT5, Fiber etc)
• Distance limitations of cable
• Remember: Most problems are caused by layer 1 issues.

     Audit current cabling to identify any areas that need to be rewired
     Put in Fiber for the backbone
     Choose the best location for the MDF and IDFs (if necessary)
     Document Layer 1
     A Logical Diagram is the network topology diagram without the exact installation 
path of the cable.
    Basic road map of LAN includes:

• Exact locations of MDF and IDF
• Type and quantity of cabling used to interconnect MDF and IDF
• How many spare cables

 A Cut Sheet is a detailed document of all cable runs, the identification numbers 
and which port they are connected to.

Step 5:
Develop the Layer two topology

The purpose of layer 2 devices -  To provide: flow control, error detection and reduce congestion
Determine the size of the collision and broadcast domains

• Remember: Besides routers, VLANs can limit collision domains

Use Switches to Microsegment to reduce the size of collision domains for improved network performance.
With a switch you can also allocate bandwidth, and you can use asymmetric switching, for more bandwidth for servers and uplinks
Allocate Bandwidth

• Remember: You wan the connections between the MDF and any IDFs to be high speed connections.

You can use switches with hubs

• Lose microsegmentation
• All workstations on hub share bandwidth
• Only one VLAN to that port on the switch

As the network grows, you can use extra cable between the MDF and IDFs to increase bandwidth.
Leave extra ports on your switches for growth.

Step 6:
Layer 3 Design

Putting routers into the network allows for control of the broadcast domain and communication across segments and VLANs
Routers: 

• Determine traffic flow based on layer 3 addressing
• Provide greater scalability by dividing the network into subnets
• Provide greater scalability by dividing the network into subnets
• Are more expensive and harder to configure then switches

Final step when implementing routers is to develop and document an IP addressing scheme

• After you have developed a addressing scheme you should document it with an addressing map. With the important hosts on a network, giving you a snapshot of the network

Create a physical map of the network, this helps with troubleshooting.