LAN design methodology

LAN design methodology 
5.1.3 For a LAN to be effective and serve the needs of its users, it should be designed and implemented based on a planned series of systematic steps. This page will describe the following steps:

• 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 process to gather information helps to clarify and identify any current network problems. This information includes the history of the organization and current status, their projected growth, operation policies and management procedures, office systems and procedures, and the viewpoints of the people who will use the LAN.
The following questions should be asked to gather information:

• Who are the people that will use the network?
• What is the skill level of these people?
• What are their attitudes toward computers and computer applications?
• How developed are the organizational documented policies?
• Has some data been declared mission critical?
• Have some operations been declared mission critical?
• What protocols are allowed on the network?
• Are only certain desktop hosts supported?
• Who is responsible for LAN addresses, naming, topology design, and configuration?
• What are the organizational human, hardware, and software resources?
• How are these resources currently linked and shared?
• What financial resources does the organization have available?

Documentation of the requirements allow for an informed estimate of costs and timelines for projected LAN design implementation. It is important to understand performance issues of any network.
Availability measures the usefulness of the network. The following are a few of the many things that affect availability:

• Throughput
• Response time
• Access to resources

Every customer has a different definition of availability. For example, there may be a need to transport voice and video over the network. These services may require more bandwidth than is available on the network or backbone. To increase availability, more resources can be added, but that increases the cost of the network. Network designs should provide the greatest availability for the least cost.
The next step in the network design is to analyze the requirements of the network and its users. Network user needs constantly change. As more voice and video-based network applications become available, the necessity to increase network bandwidth grows too.
A LAN that is not able to provide prompt and accurate information to its users is useless. Steps must be taken to ensure that the information requirements of the organization and its workers are met.
The next step is to decide on an overall LAN topology that will satisfy the user requirements.   In this curriculum, concentration will be on the star topology and extended star topology. The star topology and extended star topology use Ethernet 802.3 CSMA/CD technology. CSMA/CD star topology is the dominant configuration in the industry.
LAN topology design can be broken into the following three unique categories of the OSI reference model:

• Network layer
• Data link layer
• Physical layer

The final step in LAN design methodology is to document the physical and logical topology of the network. The physical topology of the network refers to the way in which various LAN components are connected together. The logical design of the network refers to the flow of data in a network. It also refers to the name and address schemes used in the implementation of the LAN design solution.
The following are important LAN design documentation:

• OSI layer topology map
• LAN logical map
• LAN physical map
• Cut sheets
• VLAN logical map
• Layer 3 logical map
• Address maps

The next page will discuss Layer 1 design issues.

LAN Design / LAN design goals

LAN Design 
LAN design goals 
5.1.1 The first step in LAN design is to establish and document the goals of the design. These goals are unique to each organization or situation. This page will describe the requirements of 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 not include elements 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 continuous stability of operation.

The Interactive Media Activity will help students become more familiar with the four main design goals.
The next page will discuss some LAN design considerations.

LAN design considerations

5.1.2 The four figures in this TI are useful discussion points for the class. Three key components of LAN design are placement of servers, segmentation, and bandwidth versus broadcast domain. Servers and segmentation were covered in Module 4. Bandwidth domain is everything associated with one port on a bridge or switch. The term bandwidth domain emphasizes the area of a network in which bandwidth is shared. When used in the context of an Ethernet switch, a bandwidth domain is the same as a collision domain. The best practices for teaching this TI are graphical organizers. For example, print out the four figures and have students make their own notations of collision domains, bandwidth domains, broadcast domains, and network segments. The two major categories of servers to be considered in a network design are enterprise and workgroup servers. Enterprise servers support all the users on the network through, for example, e-mail and DNS. Workgroup servers support a specific category of users such as engineers. Have the students study the layout of the network in the case study and discuss the placement of servers in this scenario and in the LAN setup at their campus. Compare this to their work in Module 4 on collision and broadcast domains.

This page will describe some important factors to consider when a LAN is designed.
Many organizations have upgraded their current LANs or plan to implement 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 domain issues
• Segmentation issues
• Broadcast domain issues

Servers allow network users to communicate, and share files, printers 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 as either enterprise servers or workgroup servers. An enterprise server supports all the users on the network as it offers services, such as e-mail or Domain Name System (DNS). E-mail or DNS is a service that everyone in an organization needs because it is a centralized function. A workgroup server supports a specific set of users and offers services such as word processing and file sharing.
As seen in Figure , enterprise servers should be placed in the main distribution facility (MDF). Whenever possible, the traffic to enterprise servers should travel only to the MDF and not be transmitted across other networks. However, some networks use a routed core or may even have a server farm for the enterprise servers. In these cases, network traffic travels across other networks and usually cannot be avoided. Ideally, workgroup servers should be placed in the intermediate distribution facilities (IDFs) closest to the users who access the applications on these servers. This allows traffic 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 collisions occur, the transmitted frame is destroyed, and a jam signal is sent to all nodes on the segment. The nodes wait a random period of time, and then resend the data. Excessive collisions can reduce the available bandwidth of a network segment to thirty-five or forty percent of the available bandwidth.
Segmentation is when a single collision domain is split into smaller collision domains. 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. Routers can achieve this at Layer 3.
A broadcast occurs when the destination media access control (MAC) 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 that originates from any device within that set. All hosts that receive a broadcast data frame must process it. This process consumes 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.
The next page will explain the methodology that should be followed for a LAN design.

Module 5: Switches / Overview

Module 5: Switches / 
Overview

 The task to design a network can be a challenge as it involves more than just a connection of two computers. A network requires many features in order to be reliable, manageable, and scalable. To design reliable, manageable, and scalable networks, network designers must realize that each of the major components of a network has distinct design requirements.
Network design has become more difficult despite improvements in equipment performance and media capabilities. The use of multiple media types and LANs that interconnect with other networks add to the complexity of the network environment. Good network designs 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 located close to each other and belong to a single organization are referred to as a campus. The following aspects of the network need to be identified before a large LAN is designed:

• An access layer that connects end users to 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 require switches that are best suited for the specific tasks. The features, functions, and technical specifications for each switch vary based on the LAN design layer for which the switch is intended. For the best network performance, it is important to understand the role of each layer and then choose the switch that best suits the layer requirements.
This module covers some of the objectives for the CCNA 640-801 and ICND 640-811 exams.  
Students who complete this module should be able to perform the following tasks:

• 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 Layers 1 through 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

Module 4 Summary

Summary
An understanding of the following key points should have been achieved:

• The history and function of shared, half-duplex Ethernet
• Collisions in an Ethernet network
• Microsegmentation
• CSMA/CD
• Elements affecting network performance
• The function of repeaters
• Network latency
• Transmission time
• The basic function of Fast Ethernet
• Network segmentation using routers, switches, and bridges
• The basic operations of a switch
• Ethernet switch latency
• The differences between Layer 2 and Layer 3 switching
• Symmetric and asymmetric switching
• Memory buffering
• Store-and-forward and cut-through switching modes
• The differences between hubs, bridges, and switches
• The main functions of switches
• Major switch frame transmission modes
• The process by which switches learn addresses
• The frame-filtering process
• LAN segmentation
• Microsegmentation using switching
• The process a switch uses to learn addresses
• Forwarding modes
• Collision and broadcast domains
• The cables needed to connect switches to workstations
• The cables needed to connect switches to switches 

This page summarizes the topics discussed in this module.

Ethernet is the most common LAN architecture and it is used to transport data between devices on a network. Originally Ethernet was a half-duplex technology. Using half-duplex, a host could either transmit or receive at one time, but not both. When two or more Ethernet hosts transmit at the same time on a shared medium, the result is a collision. The time a frame or a packet takes to travel from the source station to the final destination is known as latency or delay. The three sources of latency include NIC delay, actual propagation delay, and delay due to specific network devices.  
Bit or slot time is the basic unit of time in which ONE bit can be sent. there must be some minimum duration during which the bit is on or off in order for the device to recognize a binary one or zero.  
Attenuation means that a signal will weaken at it travels through the network. This limits the distance that a LAN can cover. A repeater can extend the distance of a LAN but it also has a negative effect on the overall performance of a LAN.  
Full-duplex transmission between stations is achieved by using point-to-point Ethernet connections. Full-duplex transmission provides a collision-free transmission environment. Both stations can transmit and receive at the same time, and there are no negotiations for bandwidth. The existing cable infrastructure can be utilized as long as the medium meets the minimum Ethernet standards.
Segmentation divides a network into smaller units to reduce network congestion and enhance security. The CSMA/CD access method on each segment maintains traffic between users. Segmentation with a Layer 2 bridge is transparent to other network devices but latency is increased significantly. The more work done by a network device, the more latency the device will introduce into the network. Routers provide segmentation of networks but can add a latency factor of 20% to 30% over a switched network. This increased latency is because a router operates at the network layer and uses the IP address to determine the best path to the destination node. A switch can segment a LAN into microsegments which decreases the size of collision domains. However all hosts connected to the switch are still in the same broadcast domain.
Switching is a technology that decreases congestion in Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI) LANs. Switching is the process of receiving an incoming frame on one interface and delivering that frame out another interface. Routers use Layer 3 switching to route a packet. Switches use Layer 2 switching to forward frames. A symmetric switch provides switched connections between ports with the same bandwidth. An asymmetric LAN switch provides switched connections between ports of unlike bandwidth, such as a combination of 10-Mbps and 100-Mbps ports.
A memory buffer is an area of memory where a switch stores data. It can use two methods for forwarding frames including port-based memory buffering and shared memory buffering.
There are two modes used to forward frames. Store-and-forward receives the entire frame before forwarding while cut-through forwards the frame as it is received decreasing latency. Fast-forward and fragment-free are two types of cut-through forwarding.

Communication between switches and workstations

Communication between switches and workstations 
4.3.9 This page will explain how switches learn about workstations in a LAN.
When a workstation connects to a LAN, it is unconcerned about the other devices that are connected to the LAN media. The workstation simply transmits data frames using a NIC to the network medium.
The workstation could be attached directly to another workstation using a crossover cable. Cross-over cables are used to connect the following devices:

• Workstation to Workstation
• Switch to Switch
• Switch to hub
• Hub to hub
• Router to router
• Router to PC

Straight-through cables are used to connect the following devices:

• Switch to router
• Switch to workstation or server
• Hub to workstation or server

Switches are Layer 2 devices that use intelligence to learn the MAC addresses of the devices that are attached to the ports of the switch. This data is entered into a switching table. Once the table is complete, the switch can read the destination MAC address of an incoming data frame on a port and immediately forward it. Until a device transmits, the switch does not know its MAC address.
Switches provide significant scalability on a network and may be directly connected. Figure illustrates one scenario of frame transmission utilizing a multi-switch network.
This page concludes this lesson. The next page will summarize the main points from this module. 

Switches and broadcast domains

Switches and broadcast domains 
4.3.8 This page will describe the three methods of data transmission that are used in a network.
Communication in a network occurs in three ways. The most common way of communication is by unicast transmissions. In a unicast transmission, one transmitter tries to reach one receiver.
Another way to communicate is known as a multicast transmission. Multicast transmission occurs when one transmitter tries to reach only a subset, or a group, of the entire segment. 
The final way to communicate is by broadcasting. Broadcasting is when one transmitter tries to reach all the receivers in the network. The server station sends out one message and everyone on that segment receives the message.
When a device wants to send out a Layer 2 broadcast, the destination MAC address in the frame is set to all ones. A MAC address of all ones is FF:FF:FF:FF:FF:FF in hexadecimal. By setting the destination to this value, all the devices will accept and process the broadcasted frame.
The broadcast domain at Layer 2 in referred to as the MAC broadcast domain. The MAC broadcast domain consists of all devices on the LAN that receive frame broadcasts by a host to all other machines on the LAN.
A switch is a Layer 2 device. When a switch receives a broadcast, it forwards it to each port on the switch except the incoming port. Each attached device must process the broadcast frame. This leads to reduced network efficiency, because available bandwidth is used for broadcasting purposes. 
When two switches are connected, the broadcast domain is increased. In this example a broadcast frame is forwarded to all connected ports on Switch 1. Switch 1 is connected to Switch 2. The frame is propagated to all devices connected to Switch 2. 
The overall result is a reduction in available bandwidth. This happens because all devices in the broadcast domain must receive and process the broadcast frame.
Routers are Layer 3 devices. Routers do not propagate broadcasts. Routers are used to segment both collision and broadcast domains.
The next page will explain how a workstation connects to a LAN.

Switches and collision domains

Switches and collision domains 
4.3.7 This page will discuss collisions, which is a major disadvantage of Ethernet 802.3 networks.
A major disadvantage of Ethernet 802.3 networks is collisions. Collisions occur when two hosts transmit frames simultaneously. When a collision occurs, the transmitted frames are corrupted or destroyed in the collision. The sending hosts stop sending further transmissions for a random period of time, based on the Ethernet 802.3 rules of CSMA/CD. Excessive collisions cause networks to be unproductive.
The network area where frames originate and collide is called the collision domain. All shared media environments are collision domains. When a host is connected to a switch port, the switch creates a dedicated connection. This connection is considered to be an individual collision domain. For example, if a twelve-port switch has a device connected to each port then twelve collision domains are created.
A switch builds a switching table by learning the MAC addresses of the hosts that are connected to each switch port. When two connected hosts want to communicate with each other, the switch looks up the switching table and establishes a virtual connection between the ports. The virtual circuit is maintained until the session is terminated.
In Figure , Host B and Host C want to communicate with each other. The switch creates the virtual connection which is referred to as a microsegment. The microsegment behaves as if the network has only two hosts, one host sending and one receiving providing maximum utilization of the available bandwidth.
Switches reduce collisions and increase bandwidth on network segments because they provide dedicated bandwidth to each network segment.
The next page will discuss three methods of data transmission in a network.