LAN Design
Overview |
The previous chapter, "VLANs," provided an introduction to virtual LANs (VLANs) and switched internetworking, compared traditional shared local-area network (LAN) configurations with switched LAN configurations, and discussed the benefits of using a switched VLAN architecture. Despite improvements in equipment performance and media capabilities, network design is becoming more difficult. The trend is toward increasingly complex environments involving multiple media and interconnection to networks outside any single organization's controlled LAN. Keeping all the many factors in mind is important because carefully designing networks can reduce the hardships associated with growth as a networking environment evolves. One of the most critical steps to ensure a fast and stable network is the design of the network. If a network is not designed properly, many unforeseen problems can arise, and network growth can be jeopardized. This design process is truly an in-depth process. This chapter provides an overview of the LAN design process. In addition, LAN design goals, network design issues, network design methodology, and the development of LAN topologies are covered in this chapter.
Washington Project: Designing the Network In this chapter, you will begin the process of designing the LAN at your specific site within Washington School District WAN. As concepts and requirements are introduced, you will be able to apply them in your network design. You will need to make sure to address the following requirements:
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4.1 LAN Design Goals and Components | |
4.1.1 LAN design goals | |
Designing a network can be a challenging task, and involves more than just connecting computers together. A network requires many features in order to be scalable and manageable. To design reliable, scalable networks, network designers must realize that each of the major components of a network has distinct design requirements. Even a network that consists of only fifty nodes can pose complex problems that lead to unpredictable results. Attempting to design and build networks that contain thousands of nodes can pose even more complex problems. The first step in designing a LAN is to establish and document the goals of the design. These goals are particular to each organization or situation. However, the following requirements tend to show up in most network designs:
These requirements are specific to certain types of networks and more general in other types of networks. This chapter discusses how to address these requirements. | |
4.1.2 Critical components of LAN design |
With the emergence of high-speed technologies such as Asynchronous Transfer Mode (ATM) and more complex LAN architectures that use LAN switching and VLANs over the past several years, many organizations have been upgrading existing LANs or planning, designing, and implementing new LANs. To design LANs for high-speed technologies and multimedia-based applications, network designers should address the following critical components of the overall LAN design:
These components are discussed in the following sections. |
4.1.3 The function and placement of servers when designing a network |
One of the keys to designing a successful network is to understand the function and placement of servers needed for the network. Servers provide file sharing, printing, communication, and application services, such as word processing. Servers typically do not function as workstations; rather, they run specialized operating systems, such as NetWare, Windows NT, UNIX, and Linux. Today, each server usually is 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 (such as the Washington School District) would need because it is a centralized function. On the other hand, a workgroup server supports a specific set of users, offering services such as word processing and file sharing, which are services only a few groups of people would need. Enterprise servers should be placed in the main distribution facility (MDF). This way, traffic to the enterprise servers has to travel only to the MDF and does not need to be transmitted across other networks. Ideally, workgroup servers should be placed in the intermediate distribution facilities (IDFs) closest to the users accessing the applications on these servers. You merely need to directly connect servers to the MDF or IDF. By placing workgroup servers close to the users, traffic only has to travel the network infrastructure to that IDF, and does not affect other users on that network segment. Within the MDF and IDFs, the Layer 2 LAN switches should have 100 Mbps or more allocated for these servers.
Washington Project: Server Placement and Function You should categorize all file servers for the Washington School District as enterprise or workgroup types, and then place servers in the network topology according to the anticipated traffic patterns of users and according to the following functions:
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4.1.4 Intranet |
One common configuration of a LAN is an intranet. Intranet Web servers differ from public Web servers in that, without the needed permissions and passwords, the public does not have access to an organization's intranet. Intranets are designed to be accessed by users who have access privileges to an organization's internal LAN. Within an intranet, Web servers are installed in the network, and browser technology is used as the common front-end to access information, such as financial data or graphical and text-based data stored on those servers. The addition of an intranet on a network is just one of many application and configuration features that can cause an increase in needed network bandwidth over current levels. Because bandwidth has to be added to the network backbone, network administrators should also consider acquiring robust desktops to get faster access into intranets. New desktops and servers should be outfitted with 10/100-Mbps Ethernet network interface cards (NICs) to provide the most configuration flexibility, thus enabling network administrators to dedicate bandwidth to individual end stations as needed. |
4.1.5 Why contention is an issue with Ethernet |
You should decide carefully on the selection and placement of networking devices to be used in the LAN in order to decrease the collision detection and media contention on a network. Contention refers to excessive collisions on Ethernet caused by too many devices, each with a great demand for the network segment. The number of broadcasts becomes excessive when there are too many client packets looking for services, too many server packets announcing services, too many routing table updates, and too many other broadcasts dependent on the protocols, such as Address Resolution Protocol (ARP). An Ethernet node gets access to the wire by contending with other Ethernet nodes for the right to do so. When your network grows to include more nodes on the shared segment or wire, and these nodes have more and more messages to transmit, the chance that a node will contend successfully for its share of the wire gets much worse, and the network bogs down. The fact that contention media access does not scale or allow for growth, is Ethernet's main disadvantage. As shown in the Figure, as traffic increases on the shared media, the rate of collisions also increases. Although collisions are normal events in Ethernet, an excessive number of collisions will (sometimes dramatically) reduce available bandwidth. In most cases, the actual available bandwidth is reduced to a fraction (about 35% to 40%) of the full 10 Mbps. This reduction in bandwidth can be remedied by segmenting the network by using bridges, switches, or routers. |
4.1.6 How broadcast domains relate to segmentation |
Segmentation is the process of splitting a single collision domain into two or more collision domains, as shown in Figure . Layer 2 (the data link layer) bridges or switches can be used to segment a logical bus topology and create separate collision domains, which results in more bandwidth being available to individual stations. Notice in Figure that the entire bus topology still represents a single broadcast domain because, although bridges and switches do not forward collisions, they forward broadcast packets. All broadcasts from any host in the same broadcast domain are visible to all other hosts in the same broadcast domain. Broadcasts must be visible to all hosts in the broadcast domain in order to establish connectivity. The scalability of the bandwidth domain depends on the total amount of traffic, and the scalability for a broadcast domain depends on the total broadcast of the traffic. It is important to remember that bridges and switches forward broadcast (FF-FF-FF-FF-FF) traffic, and that routers normally do not. |
4.1.7 The difference between bandwidth and broadcast domains |
A bandwidth domain is everything associated with one port on a bridge or switch . In the case of an Ethernet switch, a bandwidth domain is also known as a collision domain. All workstations within one bandwidth domain compete for the same LAN bandwidth resource. All the traffic from any host in the bandwidth domain is visible to all the other hosts. In the case of an Ethernet collision domain, two stations can transmit at the same time, causing a collision. |
4.2 Network Design Methodology | |
4.2.1 Gathering and analyzing requirements | |
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, which include the following:
The first step in designing a network should be to gather data about the organizational structure. This information includes the organization's history and current status, projected growth, operating policies and management procedures, office systems and procedures, and the viewpoints of the people who will be using the LAN. You need to answer the following questions: Who are the people who will be using the network? What is their level of skill, and what are their attitudes toward computers and computer applications? Answering these and similar questions will help determine how much training will be required and how many people will be needed to support the LAN. Ideally, the information gathering process helps clarify and identify the problems. You also need to determine whether there are documented policies in place. Has some data been declared mission critical? Have some operations been declared mission critical? (Mission-critical data and operations are those that are considered key to businesses, and access to them is critical to the business running on a daily basis.) What protocols are allowed on the network? Are only certain desktop hosts supported? Next, you should determine who in the organization has authority over addressing, naming, topology design, and configuration. Some companies have a central Management Information Systems (MIS) department that controls everything. Some companies have very small MIS departments and, therefore, must delegate authority to departments. Focus on identifying the resources and constraints of the organization. Organization resources that can affect the implementation of a new LAN system fall into two general categories: computer hardware/software and human resources. An organization's existing computer hardware and software must be documented, and projected hardware and software needs identified. How are these resources currently linked and shared? What financial resources does the organization have available? Documenting these types of things helps you estimate costs and develop a budget for the LAN. You should make sure you understand performance issues of any existing network.
Threaded Case Study Washington Project: Understanding the Customer First and foremost, you must understand the customer. In the case of the Washington School District, you need to talk to major users of the network; find out their geographic location, their current applications; their plans for the future; and determine who the major players will be in helping you design the network. After you have gathered data on the district's organizational structure, you need to:
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4.2.2 Factors that affect network availability |
Availability measures the usefulness of the network. Many things affect availability, including the following:
Every customer has a different definition of availability. For example, there may be a need to transport voice and video over the network. However, these services require more bandwidth than is available on the network or backbone. You can increase availability by adding more resources, but resources drive up cost. Network design seeks to provide the greatest availability for the least cost. After considering availability, the next step in designing a network is to analyze the requirements of the network and its users that were gathered in the last step. Network user needs constantly change. For example, as more voice- and video-based network applications become available, the pressure to increase network bandwidth will become intense. Another component of the analysis phase is assessing the user requirements. A LAN that is incapable of supplying prompt and accurate information to its users is of little use. Therefore, you must take steps to ensure that the information requirements of the organization and its workers are met.
Washington Project: Availability Find out what availability means to your customer. In the case of the Washington School District, you need to conduct a detailed analysis of current and projected needs in order to help meet this need. Analysis of network requirements includes analyzing the district's business and technical goals. You need to answer the following questions:
Washington Project: Determining Network Traffic Load You need to determine the network traffic load for the Washington School District before developing a network structure and acquiring hardware. Additionally, when analyzing the district's technical requirements, you should estimate the traffic load caused by applications in packet size (for example, you need to estimate the size of files in bytes per second needed to be transmitted over the network). Certain types of network use can generate large volumes of traffic and, therefore, can cause congestion, including congestion of the following:
You should estimate worst-case traffic load on the network during the busiest times for users and during regularly scheduled network services, such as file server backups. |
4.2.3 Physical topologies used in networking |
After determining the overall requirements for the network, the next step is to decide on an overall LAN topology that will satisfy the user requirements. In this curriculum, we concentrate on the star topology and extended star topology. As you have seen, the star/extended star topology uses Ethernet 802.3 carrier sense multiple access collision detect (CSMA/CD) technology. The reason that this curriculum focuses on a CSMA/CD star topology is that it is by far the dominant configuration in the industry. The major pieces of a LAN topology design can be broken into three unique categories of the OSI reference model-the network layer, the data link layer, and the physical layer. These components are discussed in the following sections. |
4.3 Layer 1 Design | |
4.3.1 Designing the layer 1 topology: signaling method, medium type, and maximum length
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In this section, you will examine Layer 1 star and extended star topologies. The physical cabling is one of the most important components to consider when designing a network. Design issues include the type of cabling to be used (typically copper or fiber) and the overall structure of the cabling. Layer 1 cabling media include types such as Category 5 unshielded twisted-pair (UTP) and fiber-optic cable, along with the TIA/EIA-568-A standard for layout and connection of wiring schemes. In addition to distance limitations, you should carefully evaluate the strengths and weaknesses of various topologies, as a network is only as effective as its underlying cable. Keep in mind that most network problems are caused by Layer 1 issues. If you are planning any significant changes for a network, you should do a complete cable audit to identify areas that require upgrades and rewiring. Whether you are designing a new network or recabling an existing one, fiber-optic cable should be used in the backbone and risers, with Category 5 UTP cable in the horizontal runs. The cable upgrade should take priority over any other needed changes, and enterprises should ensure-without exception-that these systems conform to well-defined industry standards, such as the TIA/EIA-568-A specifications. The TIA/EIA-568-A standard specifies that every device connected to the network should be linked to a central location with horizontal cabling. This is true if all the hosts that need to access the network are within the 100-meter distance limitation for Category 5 UTP Ethernet, as specified by TIA/EIA-568-A standards. The table in figure lists cable types and their characteristics. Upcoming Changes in Cabling Standards
Washington Project: Speed and Expansion For the Washington School District network, you need to build the Layer 1 components of the district network with speed and expansion capabilities. As you know, the physical layer controls the way data is transmitted between the source and a destination node. Therefore, the type of media and topology you select helps you determine how much data can travel across the network and how quickly. | |
4.3.3 HCC, VCC, MDF, IDF, and POP |
When hosts in larger networks are outside the 100-meter limitation for Category 5 UTP, it is not unusual to have more than one wiring closet. By creating multiple wiring closets, multiple catchment areas are created. The secondary wiring closets are referred to as IDFs. TIA/EIA 568-A Standards specify that IDFs should be connected to the MDF by using vertical cabling, also called backbone cabling. As shown in figure , A vertical cross-connect (VCC) is used to interconnect the various IDFs to the central MDF. Because the vertical cable lengths typically are longer than the 100-meter limit for Category 5 UTP cable, fiber-optic cabling normally is used, as shown in figure .
Washington Project: Connection Speeds In the Washington School District network, the vertical cabling should carry all data traffic between the IDFs and MDFs. Therefore, the speed of this connection should be designed to be the fast link in the network. All traffic across the district network backbone will traverse this link, so this link should be at least 100 Mbps. |
4.3.4 10BASE-T and 100BASE-TX ethernet |
Fast Ethernet is Ethernet that has been upgraded to 100 Mbps. This type uses the standard Ethernet broadcast-oriented logical bus topology of 10BASE-T, along with the familiar CSMA/CD method for Media Access Control (MAC). The Fast Ethernet standard is actually several different standards based on copper-pair wire (100BASE-TX) and on fiber-optic cable (100BASE-FX), and it is used to connect the MDF to the IDF.
Washington Project: LAN Wiring Scheme Requirements As you're planning the wiring for the sites of the Washington School District network, you need to take into account certain LAN requirements related to user access, segmentation, infrastructure, cabling, MDFs, and IDFs. Therefore, you should address the requirements described here when designing the network. Requirement 1 Requirement 2 Requirement 3 Requirement 4 |
4.3.5 Elements of a logical topology diagram |
As shown in Figure , the logical diagram is the network topology model without all the detail of the exact installation path of the cabling. It is the basic road map of the LAN. Elements of the logical diagram include:
Washington Project: Layer 2 Design Goals The following are Layer 2 LAN topology design goals for the sites of the Washington School District network:
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4.4 Explain Layer 2 Design | |
4.4.1 Common Layer 2 devices and their impact on network domains | |
As you learned previously in the "LAN Switching" chapter and in the "VLANs" chapter, the purpose of Layer 2 devices in the network is to provide flow control, error detection, error correction, and to reduce congestion in the network. The two most common Layer 2 devices (other than the NIC, which every host on the network must have) are bridges and LAN switches. Devices at this layer determine the size of the collision domains and broadcast domains. This section concentrates on the implementation of LAN switching at Layer 2.
Threaded Case Study Washington Project: Layer 2 Design goals The following are Layer 2 LAN topology design goals for the sites of the Washington School District network:
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4.4.2 Asymmetric switching |
Collisions and collision domain size are two factors that negatively affect the performance of a network. By using LAN switching, you can microsegment the network, thus eliminating collisions and reducing the size of collision domains. Another important characteristic of a LAN switch is how it can allocate bandwidth on a per-port basis, thus allowing more bandwidth to vertical cabling, uplinks, and servers. This type of switching is referred to as asymmetric switching, and it provides switched connections between ports of unlike bandwidth, such as a combination of 10-Mbps and 100-Mbps ports. |
4.4.3 The effect microsegmentation can have on a network |
As you have learned, microsegmentation means using bridges and switches to boost performance for a workgroup or a backbone. Typically, boosting performance in this manner involves Ethernet switching. Switches can be used with hubs to provide the appropriate level of performance for different users and servers. |
4.4.4 Determining the number of cable runs and drops |
By installing LAN a switch at the MDF and IDFs and vertical cable between the MDF and the IDFs, the vertical cable is carrying all the data traffic between the MDF and the IDFs. The capacity of this run must be larger than that of the runs between the IDFs and workstations. Horizontal cable runs use Category 5 UTP, and no cable drop should be longer than 100 meters, which allows links at 10 Mbps or 100 Mbps. In a normal environment, 10 Mbps is adequate for the horizontal cable drop. Because asymmetric LAN switches allow for mixing 10-Mbps and 100-Mbps ports on a single switch, the next task is to determine the number of 10-Mbps and 100- Mbps ports needed in the MDF and every IDF. This can be determined by going back to the user requirements for the number of horizontal cable drops per room and the number of drops total in any catchment area, along with the number of vertical cable runs. For example, say user requirements dictate that 4 horizontal cable runs be installed to each room. The IDF that services a catchment area covers 18 rooms. Therefore, 4 drops x 18 rooms = 72 LAN switch ports.
Washington Project: LAN Topology Requirements As you're planning the LAN topology for your school site, you need to keep in mind certain requirements for rooms that need access to the network and the room's wiring POP. Requirement 1 Requirement 2 A single location in each room needs to be designated as the wiring POP for that room. It needs to consist of a lockable cabinet containing all cable terminations and electronic components that is, data hubs or switches). From this location, data services need to be distributed within the room via decorative wire molding. Network 1 needs to be allocated for general curriculum use, and Network 2 needs to be allocated for administrative use. |
4.4.5 Determining the size of collision domains in hubbed and switched networks |
To determine the size of a collision domain, you must determine how many hosts are physically connected to any single port on the switch. This also affects how much network bandwidth is available to any host. In an ideal situation, there is only one host connected on a LAN switch port. This would make the size of the collision domain 2 (the source host and destination host). Because of this small collision domain, there should be almost no collisions when any two hosts are communicating with each other. Another way to implement LAN switching is to install shared LAN hubs on the switch ports and connect multiple hosts to a single switch port. All hosts connected to the shared LAN hub share the same collision domain and bandwidth. Note that some older switches, such as the Catalyst 1700, don't truly support sharing the same collision domain and bandwidth because they don't maintain multiple MAC addresses mapped to each port. In that case, there are many broadcasts and ARP requests. |
4.4.6 Diagramming hub placement in a standards-based extended star topology |
Shared-media hubs are generally used in a LAN switch environment to create more connection points at the end of the horizontal cable runs. This is an acceptable solution, but you must ensure that collision domains are kept small and bandwidth requirements to the host are accomplished according to specifications gathered in the requirements phase of the network design process. |
4.4.7 Migrating a network from 10 Mbps to 100 Mbps |
As the network grows, the need for more bandwidth increases. In the vertical cabling between MDF and IDFs, unused fiber optics can be connected from the VCC to 100 Mbps ports on the switch. The network shown doubles the capacity of the vertical cabling in the network in the following graphic by bringing up another link. In the horizontal cabling, you can increase the bandwidth by a factor of 10 by repatching from the HCC to a 100 Mbps port on the switch and changing from a 10 Mbps hub to a 100 Mbps hub. When sizing the Layer 2 LAN switch, it is important to make sure there are enough 100 Mbps ports to allow for this migration to higher bandwidth. It is important to document the speed at which each active cable drop is running. |
4.5 Layer 3 Design | |
4.5.1 Using routers as the basis for layer 3 network design | |
As shown in the Figure, Layer 3 (the network layer) devices, such as routers, can be used to create unique LAN segments and allow communication between segments based on Layer 3 addressing, such as IP addressing. Implementation of Layer 3 devices, such as routers, allows for segmentation of the LAN into unique physical and logical networks. Routers also allow for connectivity to wide-area networks (WANs), such as the Internet. Layer 3 routing determines traffic flow between unique physical network segments based on Layer 3 addressing, such as IP network and subnet. The router is one of the most powerful devices in the network topology. As you have learned, a router forwards data packets based on destination addresses. A router does not forward LAN-based broadcasts such as ARP requests. Therefore, the router interface is considered the entry and exit point of a broadcast domain and stops broadcasts from reaching other LAN segments.
Threaded Case Study Washington Project: Layer 3 Design Goals The following are Layer 3 LAN topology design goals for the your site:
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4.5.2 How VLANs can create smaller broadcast domains |
One important issue in a network is the total number of broadcasts, such as ARP requests. By using VLANs, you can limit broadcast traffic to within a VLAN and thus create smaller broadcast domains. VLANs can also be used to provide security by creating the VLAN groups according to function. As shown in Figure , a physical port association is used to implement VLAN assignment. Ports P0, P1, and P4 have been assigned to VLAN 1. VLAN 2 has ports P2, P3, and P5. Communication between VLAN 1 and VLAN 2 can occur only through the router. This limits the size of the broadcast domains and uses the router to determine whether VLAN 1 can talk to VLAN 2. This means you can create a security scheme based on VLAN assignment. |
4.5.3 Explain how a router provides structure to a network |
Routers provide scalability because they can serve as firewalls for broadcasts. In addition, because Layer 3 addresses typically have structure, routers can provide greater scalability by dividing networks and subnets, therefore, adding structure to Layer 3 addresses. The ways in which greater scalability in networks can occur are shown in the table. When the networks are divided into subnets, the final step is to develop and document the IP addressing scheme to be used in the network. Routing technology filters data-link broadcasts and multicasts. By adding router ports with additional subnet or network addresses, you can segment the internetwork as required. Network protocol addressing and routing provide built-in scaling. When deciding whether to use routers or switches, remember to ask, "What problem am I trying to solve?" If your problem is protocol related rather than contention oriented, then routers are appropriate. Routers solve problems with excessive broadcasts, protocols that do not scale well, security issues, and network-layer addressing. Routers, however, are more expensive and harder to configure than switches. |
4.5.4 Why large, scalable LANs need to incorporate routers |
Routers can be used to provide IP subnets to add structure to addresses. With bridges and switches, all unknown addresses must be flooded out of every port. With routers, hosts using protocols with network-layer addressing can solve the problem of finding other hosts without flooding. If the destination address is local, the sending host can encapsulate the packet in a data-link header and send a unicast frame directly to the station. The router does not see the frame and, of course, does not need to flood the frame. The sending host might have to use ARP. This would cause a broadcast, but the broadcast is only a local broadcast and is not forwarded by the router. If the destination is not local, then the sending station transmits the packet to the router. The router sends the frame to the destination or to the next hop, based on its routing table. Given this routing functionality, it is clear that large, scalable LANs need to incorporate some routers.
Washington Project: Addressing The district office should develop a complete TCP/IP addressing and naming convention scheme for all hosts, servers, and network interconnection devices. The implementation of unauthorized addresses should be prohibited. All computers located on the administrative networks should have static addresses. Curriculum computers should obtain addresses by utilizing Dynamic Host Configuration Protocol (DHCP). DHCP provides a mechanism for allocating IP addresses dynamically so that addresses can be reused when hosts no longer need them. While the district office should design, implement, and enforce the overall addressing scheme for the network, DHCP should be administered by the local sites within the confines of the address blocks they were assigned. |
4.5.5 Diagramming a standards-based LAN that uses routers |
The Figure shows an example of an implementation that has multiple physical networks. All data traffic from Network 1 destined for Network 2 has to go through the router. In this implementation, there are two broadcast domains. The two networks have unique Layer 3 IP addressing network/subnetwork addressing schemes. In a structured Layer 1 wiring scheme, multiple physical networks are easy to create simply by patching the horizontal cabling and vertical cabling into the appropriate Layer 2 switch using patch cables. As we will see in future chapters, this implementation provides for robust security implementation. In addition, the router is the central point in the LAN for traffic destination. |
4.5.6 Logical and physical network maps |
After you have developed the IP addressing scheme for the customer, you should document it by site and by network within the site. A standard convention should be set for addressing important hosts on the network. This addressing scheme should be kept consistent throughout the entire network. By creating addressing maps, you can get a snapshot of the network. Creating physical maps of the network helps you troubleshoot the network.
The following lab will help prepare for the Case Study. In this lab you will be given some basic requirements for a small LAN that spans multiple buildings. Your focus is on the physical topology and Data link layer components. The goal is to replace an aging 10BASE2 thinnet Ethernet network with current technology Ethernet switches and cabling based on structured cabling standards and the extended star topology. |
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