Switches - Module 27

                                                       Overview   

  

27.1 LAN Design   

27.1.1 LAN design goals 

27.1.2 LAN design considerations 

27.1.3 LAN design methodology 

27.1.4 Layer 1 design 

27.1.5 Layer 2 design 

27.1.6 Layer 3 design 

27.2 LAN Switches   

27.2.1 Switched LANs, access layer overview  

27.2.2 Access layer switches 

27.2.3 Distribution layer overview 

27.2.4 Distribution layer switches 

27.2.5 Core layer overview 

27.2.6 Core layer switches  

  

Summary 

  

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

  27.1  LAN Design  

  27.1.1  LAN design goals   

 

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.

  27.1  LAN Design  

  27.1.2  LAN design considerations   

 

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.

   27.1  LAN Design  

  27.1.3  LAN design methodology 

 

 

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 

  27.1  LAN Design  

  27.1.4  Layer 1 design 

 

 

This page will teach students how to design the Layer 1 topology of a network.

One of the most important components to consider in network design are the cables.  Today, most LAN cabling is based on Fast Ethernet technology. Fast Ethernet is Ethernet that has been upgraded from 10 Mbps to 100 Mbps, and has the ability to utilize full-duplex functionality. Fast Ethernet uses the standard Ethernet broadcast-oriented logical bus topology of 10BASE-T, and the CSMA/CD method for MAC addresses.

Design issues at Layer 1 include the type of cabling to be used, typically copper or fiber-optic, and the overall structure of the cabling.  This also includes the TIA/EIA-568-A standard for layout and connection of wiring schemes. Layer 1 media types include 10/100BASE-TX, Category 5, 5e, or 6 unshielded twisted-pair (UTP), or shielded twisted-pair (STP), and 100BaseFX fiber-optic cable.

Careful evaluation of the strengths and weaknesses of the topologies should be performed. A network is only as effective as the cables that are used.  Layer 1 issues cause most network problems. A complete cable audit should be conducted, when significant changes are planned for a network. This helps to identify areas that require upgrades and rewiring.

Fiber-optic cable should be used in the backbone and risers in all cable designs. Category 5e UTP cable should be used in the horizontal runs. The cable upgrade should take priority over any other necessary changes. Enterprises should also make certain 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 applies if all the hosts that need to access the network are within the 100-meter (328 ft.) distance limitation for Category 5e UTP Ethernet.

In a simple star topology with only one wiring closet, the MDF includes one or more horizontal cross-connect (HCC) patch panels.  HCC patch cables are used to connect the Layer 1 horizontal cabling with the Layer 2 LAN switch ports. The uplink port of the LAN switch, based on the model, is connected to the Ethernet port of the Layer 3 router with a patch cable. At this point, the end host has a complete physical connection to the router port.

When hosts in larger networks exceed the 100-meter (328 ft.) limitation for Category 5e UTP, more than one wiring closet is required. Multiple wiring closets mean multiple catchment areas. The secondary wiring closets are referred to as IDFs.  TIA/EIA-568-A standards specify that IDFs should be connected to the MDF by vertical cabling, also called backbone cabling.  A vertical cross-connect (VCC) is used to interconnect the various IDFs to the central MDF. Fiber-optic cable is normally used because the vertical cable lengths are typically longer than the 100-meter (328 ft.) limit for Category 5e UTP cable.

The logical diagram is the network topology model without all the details of the exact installation paths of the cables.  The logical diagram is the basic road map of the LAN which includes the following elements:

• Specify the locations and identification of the MDF and IDF wiring closets.
• Document the type and quantity of cables used to interconnect the IDFs with the MDF.
• Document the number of spare cables that are available to increase the bandwidth between the wiring closets. For example, if the vertical cabling between IDF 1 and the MDF is at eighty percent utilization, two additional pairs could be used to double the capacity.
• Provide detailed documentation of all cable runs, the identification numbers, and the port the run is terminated on at the HCC or VCC. 

The logical diagram is essential to troubleshoot network connectivity problems. If Room 203 loses connectivity to the network, the cut sheet shows that the room has cable run 203-1, which is terminated on HCC1 port 13. Cable testers can be used to determine Layer 1 failure. If it is, one of the other two runs can be used to reestablish connectivity and provide time to troubleshoot run 203-1.

  27.1  LAN Design  

  27.1.5  Layer 2 design 

 

 

This page will discuss some important Layer 2 design considerations.

The purpose of Layer 2 devices in the network is to switch frames based on destination MAC address information, provide error detection, and to reduce congestion in the network.  The two most common Layer 2 network devices are bridges and LAN switches. Devices at Layer 2 determine the size of the collision domains. 

Collisions and collision domain size are two factors that negatively affect the performance of a network.  Microsegmentation of the network reduces the size of collision domains and reduces collisions.  Microsegmentation is implemented through the use of bridges and switches. The goal is to boost performance for a workgroup or a backbone. Switches can be used with hubs to provide the appropriate level of performance for different users and servers.

Another important characteristic of a LAN switch is how it allocates bandwidth on a per-port basis. This provides more bandwidth to vertical cabling, uplinks, and servers.  This type of switching is referred to as asymmetric switching. Asymmetric switching provides switched connections between ports of unlike bandwidth, such as a combination of 10-Mbps and 100-Mbps ports. Symmetric switching provides switched connections between ports of similar bandwidth.

The desired capacity of a vertical cable run is greater than that of a horizontal cable run. The installation of a LAN switch at the MDF and IDF allows the vertical cable run to manage the data traffic from the MDF to the IDF.  The horizontal runs between the IDF and the workstations use Category 5e UTP. A horizontal cable drop should not be longer than 100 meters (328 ft.). In a normal environment, 10 Mbps is adequate for the horizontal drop. Asymmetric LAN switches allow 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 is accomplished by a review of the user requirements for the number of horizontal cable drops per room and the number of total drops in any catchment area. This includes the number of vertical cable runs. For example, suppose that user requirements dictate four horizontal cable runs to be installed in each room. The IDF services a catchment area of 18 rooms. Therefore, four drops in each of the 18 rooms equals 4x18, or 72 LAN switch ports.

The size of a collision domain is determined by the number of hosts that are physically connected to any single port on the switch. This also affects the bandwidth that is available to any host. In an ideal situation, there is only one host connected on a LAN switch port. The collision domain would consist only of the source host and destination host. The size of the collision domain would be two. Because of the small size of this collision domain, there should be virtually no collisions when any two hosts communicate with each other. Another way to implement LAN switching is to install shared LAN hubs on the switch ports. This allows multiple hosts to connect to a single switch port.  All hosts connected to the shared LAN hub share the same collision domain and bandwidth. That means that collisions would occur more frequently.

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 care must be taken. Collision domains should be kept small and bandwidth to the host must be provided in accordance to the specifications gathered in the requirements phase of the network design process.

     27.1  LAN Design 

27.1.6  Layer 3 design    

 

This page will describe some Layer 3 design considerations.

A router is a Layer 3 device and is considered one of the most powerful devices in the network topology.

Layer 3 devices can be used to create unique LAN segments. Layer 3 devices allow communication between segments based on Layer 3 addresses, such as IP addresses. Implementation of Layer 3 devices allows for segmentation of the LAN into unique physical and logical networks. Routers also allow for connectivity to WANs, such as the Internet.

Layer 3 routing determines traffic flow between unique physical network segments based on Layer 3 addresses. 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 to other LAN segments.

Routers provide scalability because they serve as firewalls for broadcasts and they can divide networks into subnetworks, or subnets, based on Layer 3 addresses.

In order to decide whether to use routers or switches, it is important to determine the problem that needs to be solved. If the problem is related to protocol rather than issues of contention, then routers are the appropriate solution. Routers solve problems with excessive broadcasts, protocols that do not scale well, security issues, and network layer addresses. Routers are more expensive and more difficult to configure than switches.

Figure  shows an example of an implementation that has multiple 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 network address schemes. Multiple physical networks can be created if the horizontal cabling and vertical cabling are patched into the appropriate Layer 2 switch. This can be done with patch cables. This implementation also provides robust security because all traffic in and out of the LAN must pass through the router.

Once an IP address scheme is developed for a client, it should be clearly documented. A standard convention should be set for addresses of important hosts on the network.  This address scheme should be kept consistent throughout the entire network. Address maps provide a snapshot of the network.    Physical maps of the network helps to troubleshoot the network.

VLAN implementation combines Layer 2 switching and Layer 3 routing technologies to limit both collision domains and broadcast domains. VLANs also provide security with the creation of VLAN groups that communicate with other VLANs through routers.

A physical port association is used to implement VLAN assignment. Ports P1, P4, and P6 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.

  27.2  LAN Switches  

  27.2.1  Switched LANs, access layer overview 

 

 

The construction of a LAN that satisfies the needs of both medium and large-sized organizations is more likely to be successful if a hierarchical design model is used. The use of a hierarchical design model will make it easier to make changes to the network as the organization grows. This page will discuss the three layers of the hierarchical design model:

• The access layer provides users in workgroups access to the network. 
• The distribution layer provides policy-based connectivity.
• The core layer provides optimal transport between sites. The core layer is often referred to as the backbone.

This hierarchical model applies to any network design. It is important to realize that these three layers may exist in clear and distinct physical entities. However, this is not a requirement. These layers are defined to aid in successful network design and to represent functionality that must exist in a network.

The access layer is the entry point for user workstations and servers to the network. In a campus LAN the device used at the access layer can be a switch or a hub.

If a hub is used, bandwidth is shared. If a switch is used, then bandwidth is dedicated. If a workstation or server is directly connected to a switch port, then the full bandwidth of the connection to the switch is available to the connected computer. If a hub is connected to a switch port, bandwidth is shared between all devices connected to the hub.

Access layer functions also include MAC layer filtering and microsegmentation. MAC layer filtering allows switches to direct frames only to the switch port that is connected to the destination device. The switch creates small Layer 2 segments called microsegments. The collision domain can be as small as two devices. Layer 2 switches are used in the access layer.

  27.2  LAN Switches  

  27.2.2  Access layer switches

 

 

This page will explain the functions of access layer switches.

Access layer switches operate at Layer 2 of the OSI model and provide services such as VLAN membership. The main purpose of an access layer switch is to allow end users into the network. An access layer switch should provide this functionality with low cost and high port density.

The following Cisco switches are commonly used at the access layer:

• Catalyst 1900 series
• Catalyst 2820 series
• Catalyst 2950 series
• Catalyst 4000 series
• Catalyst 5000 series

The Catalyst 1900 or 2820 series switch is an effective access device for small or medium campus networks. The Catalyst 2950 series switch effectively provides access for servers and users that require higher bandwidth. This is achieved with Fast Ethernet capable switch ports. The Catalyst 4000 and 5000 series switches include Gigabit Ethernet ports and are effective access devices for a larger number of users in large campus networks.

  27.2  LAN Switches  

  27.2.3  Distribution layer overview   

 

This page will describe the distribution layer and explain its purpose.

The distribution layer of the network is between the access and core layers. It helps to define and separate the core. The purpose of this layer is to provide a boundary definition in which packet manipulation can take place. Networks are segmented into broadcast domains by this layer. Policies can be applied and access control lists can filter packets. The distribution layer does not allow the problems to affect the core layer. The distribution layer also prevents these problems from affecting the core layer. Switches in this layer operate at Layer 2 and Layer 3. The following are some of the distribution layer functions in a switched network:

• Aggregation of the wiring closet connections
• Broadcast/multicast domain definition
• VLAN routing
• Any media transitions that need to occur
• Security

  27.2  LAN Switches  

  27.2.4  Distribution layer switches    

 

This page will explain the features and functions of distribution layer switches.

Distribution layer switches are the aggregation points for multiple access layer switches. The switch must be able to accommodate the total amount of traffic from the access layer devices.

The distribution layer switch must have high performance. The distribution layer switch is a point at which a broadcast domain is delineated. The distribution layer combines VLAN traffic and is a focal point for policy decisions about traffic flow. For these reasons, distribution layer switches operate at both Layer 2 and Layer 3 of the OSI model. Switches in this layer are referred to as multilayer switches. These multilayer switches combine the functions of a router and a switch in one device. They are designed to switch traffic to gain higher performance than a standard router. If they do not have an associated router module, then an external router is used for the Layer 3 function.

The following Cisco switches are suitable for the distribution layer: 

• Catalyst 2926G 
• Catalyst 5000 family
• Catalyst 6000 family   

  27.2  LAN Switches  

  27.2.5  Core layer overview     

 

This page will discuss the main functions of the core layer.

The core layer is a high-speed switching backbone. If they do not have an associated router module, an external router is used for the Layer 3 function. This layer of the network design should not perform any packet manipulation. Packet manipulation, such as access list filtering, would slow down the switching of packets. A core infrastructure with redundant alternate paths gives stability to the network in the event of a single device failure.

The core can be designed to use Layer 2 or Layer 3 switching. ATM or Ethernet switches can be used.

The Interactive Media Activity will require students to identify the main functions of the access, distribution, and core layers.

  27.2  LAN Switches  

  27.2.6  Core layer switches    

 

This page will explain the basic requirements for core layer switches.

The core layer is the backbone of the campus switched network. The switches in this layer can make use of a number of Layer 2 technologies. Provided that the distance between the core layer switches is not too great, the switches can use Ethernet technology. Other Layer 2 technologies such as ATM cell switching, can also be used. In a network design, the core layer can be a routed, or Layer 3, core. Core layer switches are designed to provide efficient Layer 3 functionality when needed. Factors such as need, cost, and performance should be considered before a choice is made.

The following Cisco switches are suitable for the core layer:  -

• Catalyst 6500 series
• Catalyst 8500 series
• IGX 8400 series
• Lightstream 1010

  Summary

 

 

This page summarizes the topics discussed in this module.

LAN design depends on the requirements of individual organizations but typically focuses on functionality, scalability, manageability, and adaptability. For a LAN to be effective, it should be designed and implemented based on a planned series of systematic steps. The steps require data and requirements to be gathered and analyzed, Layers 1,2, and 3 implemented, and everything to be documented. 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

Layer 1 design issues include the type of cables to be used and the overall structure of the cabling. This also includes the TIA/EIA-568-A standard for layout and connection of wiring schemes. Layer 1 media types include 10/100BASE-TX, Category 5, 5e, or 6 unshielded twisted-pair (UTP), or shielded twisted-pair (STP), and 100BaseFX fiber-optic cable.

The logical diagram of the LAN includes the locations and identification of the MDF and IDF wiring closets, the type and quantity of cables used to interconnect the IDFs with the MDF, and the number of spare cables available to increase the bandwidth between the wiring closets.

Layer 2 devices provide flow control, error detection, error correction, and reduce congestion in the network. Bridges and LAN switches are the two most common Layer 2 network devices. Microsegmentation of the network reduces the size of collision domains and reduces collisions.

Routers are Layer 3 devices that can be used to create unique LAN segments. They allow communication between segments based on Layer 3 addresses, such as IP addresses. Implementation of Layer 3 devices allows for segmentation of the LAN into unique physical and logical networks. Routers also allow for connectivity to WANs such as the Internet.

VLAN implementation combines Layer 2 switching and Layer 3 routing technologies to limit both collision domains and broadcast domains. VLANs can also be used to provide security by creating the VLAN groups according to function and by using routers to communicate between VLANs.

The hierarchical design model includes three layers. The access layer provides users in workgroups, access to the network. The distribution layer provides policy-based connectivity. The core layer provides optimal transport between sites. The core layer is often referred to as the backbone.

Access layer switches operate at Layer 2 of the OSI model and provide services such as VLAN membership. The main purpose of an access layer switch is to allow end users into the network. An access layer switch should provide this functionality with low cost and high port density.

The distribution layer switch is a point at which a broadcast domain is delineated. The distribution layer combines VLAN traffic and is a focal point for policy decisions about traffic flow. For these reasons, distribution layer switches operate at both Layer 2 and Layer 3 of the OSI model. Switches in this layer are referred to as multilayer switches.

The core layer is a high-speed switching backbone. This layer of the network design should not perform any packet manipulation. Packet manipulation, such as access list filtering, would slow down the switching of packets. A core infrastructure with redundant alternate paths give stability to the network in the event of a single device failure.