CCNA 3: LAN SWITCHING AND WIRELESS
CHAPTER 1: LAN DESIGN
For the small- and medium-sized business, communicating digitally using data, voice, and video is critical to business survival. Consequently, a properly designed LAN is a fundamental requirement for doing business today. You must be able to recognize a well-designed LAN and select the appropriate devices to support the network specifications of a small- or medium-sized business.
In this chapter, you will begin exploring the switched LAN architecture and some of the principles that are used to design a hierarchical network. You will learn about converged networks. You will also learn how to select the correct switch for a hierarchal network and which Cisco switches are best suited for each network layer. The activities and labs confirm and reinforce your learning.
1.1 SWITCHED LAN ARCHITECTURE
THE HIERARCHICAL NETWORK MODEL
Figure 1-1: The Hierarchical Network Model
When building a LAN that satisfies the needs of a small- or medium-sized business, your plan is more likely to be successful if a hierarchical design model is used. Compared to other network designs, a hierarchical network is easier to manage and expand, and problems are solved more quickly.
Hierarchical network design involves dividing the network into discrete layers. Each layer provides specific functions that define its role within the overall network. By separating the various functions that exist on a network, the network design becomes modular, which facilitates scalability and performance. The typical hierarchical design model is broken up in to three layers: access, distribution, and core. An example of a three-layer hierarchical network design is displayed in Figure 1-1.
The access layer interfaces with end devices, such as PCs, printers, and IP phones, to provide access to the rest of the network. The access layer can include routers, switches, bridges, hubs, and wireless access points. The main purpose of the access layer is to provide a means of connecting devices to the network and controlling which devices are allowed to communicate on the network.
The distribution layer aggregates the data received from the access layer switches before it is transmitted to the core layer for routing to its final destination. The distribution layer controls the flow of network traffic using policies and delineates broadcast domains by performing routing functions between Virtual LANs (VLANs) defined at the access layer. VLANs allow you to segment the traffic on a switch into separate subnetworks. For example, in a university you might separate traffic according to faculty, students, and guests. Distribution layer switches are typically high-performance devices that have high availability and redundancy to ensure reliability. You will learn more about VLANs, broadcast domains, and inter-VLAN routing later in this course.
The core layer of the hierarchical design is the high-speed backbone of the internetwork. The core layer is critical for interconnectivity between distribution layer devices, so it is important for the core to be highly available and redundant. The core area can also connect to Internet resources. The core aggregates the traffic from all the distribution layer devices, so it must be capable of forwarding large amounts of data quickly.
Note: In smaller networks, it is not unusual to implement a collapsed core model, where the distribution layer and core layer are combined into one layer.
II. A Hierarchical Network in a Medium-Sized Business
Let us look at the hierarchical network model applied to a business. In the Hierarchical Network Model, the access, distribution, and core layers are separated into a well-defined hierarchy. This logical representation makes it easy to see which switches perform which function. It is much harder to see these hierarchical layers when the network is installed in a business.
Figure 1-2: Physical Layout
The figure shows two floors of a building. The user computers and network devices that need network access are on one floor. The resources, such as e-mail servers and database servers, are located on another floor. To ensure that each floor has access to the network, access layer and distribution switches are installed in the wiring closets of each floor and connected to each of the devices needing network access. The figure shows a small rack of switches. The access layer switch and distribution layer switch are stacked one on top of each other in the wiring closet.
Although the core and other distribution layer switches are not shown, you can see how the physical layout of a network differs from the logical layout of a network.
III. Benefits of a Hierarchical Network
There are many benefits associated with hierarchical network designs.
Hierarchical networks scale very well. The modularity of the design allows you to replicate design elements as the network grows. Because each instance of the module is consistent, expansion is easy to plan and implement. For example, if your design model consists of two distribution layer switches for every 10 access layer switches, you can continue to add access layer switches until you have 10 access layer switches cross-connected to the two distribution layer switches before you need to add additional distribution layer switches to the network topology. Also, as you add more distribution layer switches to accommodate the load from the access layer switches, you can add additional core layer switches to handle the additional load on the core.
As a network grows, availability becomes more important. You can dramatically increase availability through easy redundant implementations with hierarchical networks. Access layer switches are connected to two different distribution layer switches to ensure path redundancy. If one of the distribution layer switches fails, the access layer switch can switch to the other distribution layer switch. Additionally, distribution layer switches are connected to two or more core layer switches to ensure path availability if a core switch fails. The only layer where redundancy is limited is at the access layer. Typically, end node devices, such as PCs, printers, and IP phones, do not have the ability to connect to multiple access layer switches for redundancy. If an access layer switch fails, just the devices connected to that one switch would be affected by the outage. The rest of the network would continue to function unaffected.
Communication performance is enhanced by avoiding the transmission of data through low-performing, intermediary switches. Data is sent through aggregated switch port links from the access layer to the distribution layer at near wire speed in most cases. The distribution layer then uses its high performance switching capabilities to forward the traffic up to the core, where it is routed to its final destination. Because the core and distribution layers perform their operations at very high speeds, there is no contention for network bandwidth. As a result, properly designed hierarchical networks can achieve near wire speed between all devices.
Security is improved and easier to manage. Access layer switches can be configured with various port security options that provide control over which devices are allowed to connect to the network. You also have the flexibility to use more advanced security policies at the distribution layer. You may apply access control policies that define which communication protocols are deployed on your network and where they are permitted to go. For example, if you want to limit the use of HTTP to a specific user community connected at the access layer, you could apply a policy that blocks HTTP traffic at the distribution layer. Restricting traffic based on higher layer protocols, such as IP and HTTP, requires that your switches are able to process policies at that layer. Some access layer switches support Layer 3 functionality, but it is usually the job of the distribution layer switches to process Layer 3 data, because they can process it much more efficiently.
Manageability is relatively simple on a hierarchical network. Each layer of the hierarchical design performs specific functions that are consistent throughout that layer. Therefore, if you need to change the functionality of an access layer switch, you could repeat that change across all access layer switches in the network because they presumably perform the same functions at their layer. Deployment of new switches is also simplified because switch configurations can be copied between devices with very few modifications. Consistency between the switches at each layer allows for rapid recovery and simplified troubleshooting. In some special situations, there could be configuration inconsistencies between devices, so you should ensure that configurations are well documented so that you can compare them before deployment.
Because hierarchical networks are modular in nature and scale very easily, they are easy to maintain. With other network topology designs, manageability becomes increasingly complicated as the network grows. Also, in some network design models, there is a finite limit to how large the network can grow before it becomes too complicated and expensive to maintain. In the hierarchical design model, switch functions are defined at each layer, making the selection of the correct switch easier. Adding switches to one layer does not necessarily mean there will not be a bottleneck or other limitation at another layer. For a full mesh network topology to achieve maximum performance, all switches need to be high-performance switches, because each switch needs to be capable of performing all the functions on the network. In the hierarchical model, switch functions are different at each layer. You can save money by using less expensive access layer switches at the lowest layer, and spend more on the distribution and core layer switches to achieve high performance on the network.
PRINCIPLES OF HIERARCHICAL NETWORK DESIGN
Hierarchical Network Design Principles
Just because a network seems to have a hierarchical design does not mean that the network is well designed. These simple guidelines will help you differentiate between well-designed and poorly designed hierarchical networks. This section is not intended to provide you with all the skills and knowledge you need to design a hierarchical network, but it offers you an opportunity to begin to practice your skills by transforming a flat network topology into a hierarchical network topology.
1. Network Diameter
When designing a hierarchical network topology, the first thing to consider is network diameter. Diameter is usually a measure of distance, but in this case, we are using the term to measure the number of devices. Network diameter is the number of devices that a packet has to cross before it reaches its destination. Keeping the network diameter low ensures low and predictable latency between devices.
Each switch in the path introduces some degree of latency. Network device latency is the time spent by a device as it processes a packet or frame. Each switch has to determine the destination MAC address of the frame, check its MAC address table, and forward the frame out the appropriate port. Even though that entire process happens in a fraction of a second, the time adds up when the frame has to cross many switches.
In the three-layer hierarchical model, Layer 2 segmentation at the distribution layer practically eliminates network diameter as an issue. In a hierarchical network, network diameter is always going to be a predictable number of hops between the source and destination devices.
2. Bandwidth Aggregation
Each layer in the hierarchical network model is a possible candidate for bandwidth aggregation. Bandwidth aggregation is the practice of considering the specific bandwidth requirements of each part of the hierarchy. After bandwidth requirements of the network are known, links between specific switches can be aggregated, which is called link aggregation. Link aggregation allows multiple switch port links to be combined so as to achieve higher throughput between switches. Cisco has a proprietary link aggregation technology called EtherChannel, which allows multiple Ethernet links to be consolidated. A discussion of EtherChannel is beyond the scope of this course. To learn more, visit: http://www.cisco.com/en/US/tech/tk389/tk213/tsd_technology_support_protocol_home.html.
Redundancy is one part of creating a highly available network. Redundancy can be provided in a number of ways. For example, you can double up the network connections between devices, or you can double the devices themselves. This chapter explores how to employ redundant network paths between switches. A discussion on doubling up network devices and employing special network protocols to ensure high availability is beyond the scope of this course. For an interesting discussion on high availability, visit: http://www.cisco.com/en/US/products/ps6550/products_ios_technology_home.html
Implementing redundant links can be expensive. Imagine if every switch in each layer of the network hierarchy had a connection to every switch at the next layer. It is unlikely that you will be able to implement redundancy at the access layer because of the cost and limited features in the end devices, but you can build redundancy into the distribution and core layers of the network.
Figure 1-3: Redundancy
In the figure, redundant links are shown at the distribution layer and core layer. At the distribution layer, there are two distribution layer switches, the minimum required to support redundancy at this layer. The access layer switches, S1, S3, S4, and S6, are cross-connected to the distribution layer switches. This protects your network if one of the distribution switches fails. In case of a failure, the access layer switch adjusts its transmission path and forwards the traffic through the other distribution switch.
Some network failure scenarios can never be prevented, for example, if the power goes out in the entire city, or the entire building is demolished because of an earthquake. Redundancy does not attempt to address these types of disasters. To learn more about how a business can continue to work and recover from a disaster, visit: http://www.cisco.com/en/US/netsol/ns516/networking_solutions_package.html
Start at the Access Layer
Imagine that a new network design is required. Design requirements, such as the level of performance or redundancy necessary, are determined by the business goals of the organization. Once the design requirements are documented, the designer can begin selecting the equipment and infrastructure to implement the design.
When you start the equipment selection at the access layer, you can ensure that you accommodate all network devices needing access to the network. After you have all end devices accounted for, you have a better idea of how many access layer switches you need. The number of access layer switches, and the estimated traffic that each generates, helps you to determine how many distribution layer switches are required to achieve the performance and redundancy needed for the network. After you have determined the number of distribution layer switches, you can identify how many core switches are required to maintain the performance of the network.
A thorough discussion on how to determine which switch to select based on traffic flow analysis and how many core switches are required to maintain performance is beyond the scope of this course. For a good introduction to network design, read this book that is available from Ciscopress.com: Top-Down Network Design, by Priscilla Oppenheimer (2004).
WHAT IS A CONVERGED NETWORK?
Small and medium-sized businesses are embracing the idea of running voice and video services on their data networks. Let us look at how voice and video over IP (VoIP) affect a hierarchical network.
Convergence is the process of combining voice and video communications on a data network. Converged networks have existed for a while now, but were only feasible in large enterprise organizations because of the network infrastructure requirements and complex management that was involved to make them work seamlessly. There were high network costs associated with convergence because more expensive switch hardware was required to support the additional bandwidth requirements. Converged networks also required extensive management in relation to Quality of Service (QoS), because voice and video data traffic needed to be classified and prioritized on the network. Few individuals had the expertise in voice, video, and data networks to make convergence feasible and functional. In addition, legacy equipment hinders the process. Most telephone companies today have made the transition to digital-based switches. However, there are many offices that still use analog phones, so they still have existing analog telephone wiring closets. Because analog phones have not yet been replaced, you will also see equipment that has to support both legacy PBX telephone systems and IP-based phones. This sort of equipment will slowly be migrated to modern IP-based phone switches.
Converging voice, video, and data networks has become more popular recently in the small to medium-sized business market because of advancements in technology. Convergence is now easier to implement and manage, and less expensive to purchase.
Moving to a converged network can be a difficult decision if the business already invested in separate voice, video, and data networks. It is difficult to abandon an investment that still works, but there are several advantages to converging voice, video, and data on a single network infrastructure.
One benefit of a converged network is that there is just one network to manage. With separate voice, video, and data networks, changes to the network have to be coordinated across networks. There are also additional costs resulting from using three sets of network cabling. Using a single network means you just have to manage one wired infrastructure.
Another benefit is lower implementation and management costs. It is less expensive to implement a single network infrastructure than three distinct network infrastructures. Managing a single network is also less expensive. Traditionally, if a business has a separate voice and data network, they have one group of people managing the voice network and another group managing the data network. With a converged network, you have one group managing both the voice and data networks.
Converged networks give you options that had not existed previously. You can now tie voice and video communications directly into an employee's personal computer system. There is no need for expensive handset phones or videoconferencing equipment. You can accomplish the same function using special software integrated with a personal computer. Softphones, such as the Cisco IP Communicator, offer a lot of flexibility for businesses. When software is used in place of a physical phone, a business can quickly convert to converged networks, because there is no capital expense in purchasing IP phones and the switches needed to power the phones. With the addition of inexpensive webcams, videoconferencing can be added to a softphone. These are just a few examples provided by a broader communications solution portfolio that redefine business processes today.
II. Separate Voice, Video and Data Networks
A voice network contains isolated phone lines running to a PBX switch to allow phone connectivity to the Public Switched Telephone Network (PSTN). When a new phone is added, a new line has to be run back to the PBX. The PBX switch is typically located in a Telco wiring closet, separate from the data and video wiring closets. The wiring closets are usually separated because different support personnel require access to each system. However, using a properly designed hierarchical network, and implementing QoS policies that prioritize the audio data, voice data can be converged onto an existing data network with little to no impact on audio quality.
Videoconferencing equipment is wired separately from the voice and data networks. Videoconferencing data can consume significant bandwidth on a network. As a result, video networks were maintained separately to allow the videoconferencing equipment to operate at full speed without competing for bandwidth with voice and data streams. Using a properly designed hierarchical network, and implementing QoS policies that prioritize the video data, video can be converged onto an existing data network with little to no impact on video quality.
The data network interconnects the workstations and servers on a network to facilitate resource sharing. Data networks can consume significant data bandwidth, which is why voice, video, and data networks were kept separated for such a long time. Now that properly designed hierarchical networks can accommodate the bandwidth requirements of voice, video, and data communications at the same time, it makes sense to converge them all onto a single hierarchical network.
CONSIDERATIONS FOR HIERARCHICAL NETWORK SWITCHES
Traffic Flow Analysis
To select the appropriate switch for a layer in a hierarchical network, you need to have specifications that detail the target traffic flows, user communities, data servers, and data storage servers.
Companies need a network that can meet evolving requirements. A business may start with a few PCs interconnected so that they can share data. As the business adds more employees, devices, such as PCs, printers, and servers, are added to the network. Accompanying the new devices is an increase in network traffic. Some companies are replacing their existing telephone systems with converged VoIP phone systems, which adds additional traffic.
When selecting switch hardware, determine which switches are needed in the core, distribution, and access layers to accommodate the bandwidth requirements of your network. Your plan should take into account future bandwidth requirements. Purchase the appropriate Cisco switch hardware to accommodate both current needs as well as future needs. To help you more accurately choose appropriate switches, perform and record traffic flow analyses on a regular basis.
Traffic flow analysis is the process of measuring the bandwidth usage on a network and analyzing the data for the purpose of performance tuning, capacity planning, and making hardware improvement decisions. Traffic flow analysis is done using traffic flow analysis software. Although there is no precise definition of network traffic flow, for the purposes of traffic flow analysis we can say that network traffic is the amount of data sent through a network for a given period of time. All network data contributes to the traffic, regardless of its purpose or source. Analyzing the various traffic sources and their impact on the network, allows you to more accurately tune and upgrade the network to achieve the best possible performance.
Traffic flow data can be used to help determine just how long you can continue using existing network hardware before it makes sense to upgrade to accommodate additional bandwidth requirements. When you are making your decisions about which hardware to purchase, you should consider port densities and switch forwarding rates to ensure adequate growth capability. Port density and forwarding rates are explained later in this chapter.
There are many ways to monitor traffic flow on a network. You can manually monitor individual switch ports to get the bandwidth utilization over time. When analyzing the traffic flow data, you want to determine future traffic flow requirements based on the capacity at certain times of the day and where most of the data is generated and sent. However, to obtain accurate results, you need to record enough data. Manual recording of traffic data is a tedious process that requires a lot of time and diligence. Fortunately, there are some automated solutions.
Figure 1-4: Output of Solarwinds Orion 8.1 NetFlow
Many traffic flow analysis tools that automatically record traffic flow data to a database and perform a trend analysis are available. In larger networks, software collection solutions are the only effective method for performing traffic flow analysis. An example of an Analysis Tool is Solarwinds Orion 8.1 NetFlow Analysis. While the software is collecting data, you can see just how every interface is performing at any given point in time on the network. Using the included charts, you can identify traffic flow problems visually. This is much easier than having to interpret the numbers in a column of traffic flow data.
For a list of some commercial traffic flow collection and analysis tools, visit http://www.cisco.com/warp/public/732/Tech/nmp/netflow/partners/commercial/index.shtml.
For a list of some freeware traffic flow collection and analysis tools, visit http://www.cisco.com/warp/public/732/Tech/nmp/netflow/partners/freeware/index.shtml.
II. User Communities Analysis
User community analysis is the process of identifying various groupings of users and their impact on network performance. The way users are grouped affects issues related to port density and traffic flow, which, in turn, influences the selection of network switches. Port density is explained later in this chapter.
In a typical office building, end users are grouped according to their job function, because they require similar access to resources and applications. You may find the Human Resource (HR) department located on one floor of an office building, while Finance is located on another floor. Each department has a different number of users and application needs, and requires access to different data resources available through the network. For example, when selecting switches for the wiring closets of the HR and Finance departments, you would choose a switch that had enough ports to meet the department needs and was powerful enough to accommodate the traffic requirements for all the devices on that floor. Additionally, a good network design plan factors in the growth of each department to ensure that there are enough open switch ports that can utilized before the next planned upgrade to the network.
But this plan does not account for future growth. Consider what will happen if the HR department grows by five employees. A solid network plan includes the rate of personnel growth over the past five years to be able to anticipate the future growth. With that in mind, you would want to purchase a switch that can accommodate more than 24 ports, such as stackable or modular switches that can scale.
As well as looking at the number of devices on a given switch in a network, you should investigate the network traffic generated by end-user applications. Some user communities use applications that generate a lot of network traffic, while other user communities do not. By measuring the network traffic generated for all applications in use by different user communities, and determining the location of the data source, you can identify the effect of adding more users to that community.
A workgroup-sized user community in a small business is supported by a couple of switches and typically connected to the same switch as the server. In medium-sized businesses or enterprises, user communities are supported by many switches. The resources that medium-sized business or enterprise user communities need could be located in geographically separate areas. Consequently, the location of the user communities influences where data stores and server farms are located.
If the Finance users are using a network-intensive application that exchanges data with a specific server on the network, it may make sense to locate the Finance user community close to that server. By locating users close to their servers and data stores, you can reduce the network diameter for their communications, thereby reducing the impact of their traffic across the rest of the network.
One complication of analyzing application usage by user communities is that usage is not always bound by department or physical location. You may have to analyze the impact of the application across many network switches to determine its overall impact.
III. Data Stores and Data Servers Analysis
When analyzing traffic on a network, consider where the data stores and servers are located so that you can determine the impact of traffic on the network. Data stores can be servers, storage area networks (SANs), network-attached storage (NAS), tape backup units, or any other device or component where large quantities of data are stored.
When considering the traffic for data stores and servers, consider both client-server traffic and server-server traffic.
Client-server traffic is the traffic generated when a client device accesses data from data stores or servers. Client-server traffic typically traverses multiple switches to reach its destination. Bandwidth aggregation and switch forwarding rates are important factors to consider when attempting to eliminate bottlenecks for this type of traffic.
Server-server traffic is the traffic generated between data storage devices on the network. Some server applications generate very high volumes of traffic between data stores and other servers. To optimize server-server traffic, servers needing frequent access to certain resources should be located in close proximity to each other so that the traffic they generate does not affect the performance of the rest of the network. Servers and data stores are typically located in data centers within a business. A data center is a secured area of the building where servers, data stores, and other network equipment are located. A device can be physically located in the data center but represented in quite a different location in the logical topology. Traffic across data center switches is typically very high due to the server-server and client-server traffic that traverses the switches. As a result, switches selected for data centers should be higher performing switches than the switches you would find in the wiring closets at the access layer.
By examining the data paths for various applications used by different user communities, you can identify potential bottlenecks where performance of the application can be affected by inadequate bandwidth. To improve the performance, you could aggregate links to accommodate the bandwidth, or replace the slower switches with faster switches capable of handling the traffic load.
IV. Topology Diagrams
A topology diagram is a graphical representation of a network infrastructure. A topology diagram shows how all switches are interconnected, detailed down to which switch port interconnects the devices. A topology diagram graphically displays any redundant paths or aggregated ports between switches that provide for resiliency and performance. It shows where and how many switches are in use on your network, as well as identifies their configuration. Topology diagrams can also contain information about device densities and user communities. Having a topology diagram allows you to visually identify potential bottlenecks in network traffic so that you can focus your traffic analysis data collection on areas where improvements can have the most significant impact on performance.
A network topology can be very difficult to piece together after the fact if you were not part of the design process. Network cables in the wiring closets disappear into the floors and ceilings, making it difficult to trace their destinations. And because devices are spread throughout the building, it is difficult to know how all of the pieces are connected together. With patience, you can determine just how everything is interconnected and then document the network infrastructure in a topology diagram.
The topology diagram identifies each switch port used for inter-switch communications and redundant paths between access layer switches and distribution layer switches. The topology diagram also displays where different user communities are located on the network and the location of the servers and data stores.
I. Switch Form Factors
What are the key features of switches that are used in hierarchical networks? When you look up the specifications for a switch, what do all of the acronyms and word phrases mean? What does "PoE" mean and what is "forwarding rate"? In this topic, you will learn about these features.
When you are selecting a switch, you need to decide between fixed configuration and modular configuration, and stackable or non-stackable. Another consideration is the thickness of the switch expressed in number of rack units. For example, the Fixed Configuration Switches are all 1 rack unit (1U). These options are sometimes referred to as switch form factors.
Fixed Configuration Switches
Fixed configuration switches are just as you might expect, fixed in their configuration. What that means is that you cannot add features or options to the switch beyond those that originally came with the switch. The particular model you purchase determines the features and options available. For example, if you purchase a 24-port gigabit fixed switch, you cannot add additional ports when you need them. There are typically different configuration choices that vary in how many and what types of ports are included.
Figure 1-5: Fixed Configuration Switches
Modular switches offer more flexibility in their configuration. Modular switches typically come with different sized chassis that allow for the installation of different numbers of modular line cards. The line cards actually contain the ports. The line card fits into the switch chassis like expansion cards fit into a PC. The larger the chassis, the more modules it can support. If you bought a modular switch with a 24-port line card, you could easily add an additional 24 port line card, to bring the total number of ports up to 48.
Figure 1-6: Modular Switches
Figure 1-7: Stackable Switches
Stackable switches can be interconnected using a special backplane cable that provides high-bandwidth throughput between the switches. Cisco introduced StackWise technology in one of its switch product lines. StackWise allows you to interconnect up to nine switches using fully redundant backplane connections. As you can see in the figure, switches are stacked one atop of the other, and cables connect the switches in daisy chain fashion. The stacked switches effectively operate as a single larger switch. Stackable switches are desirable where fault tolerance and bandwidth availability are critical and a modular switch is too costly to implement. Using cross-connected connections, the network can recover quickly if a single switch fails. Stackable switches use a special port for interconnections and do not use line ports for inter-switch connections. The speeds are also typically faster than using line ports for connection switches.
When selecting a switch for the access, distribution, or core layers, consider the ability of the switch to support the port density, forwarding rates, and bandwidth aggregation requirements of your network.
Port density is the number of ports available on a single switch. Fixed configuration switches typically support up to 48 ports on a single device, with options for up to four additional ports for small form-factor pluggable (SFP) devices, as shown in the figure. High port densities allow for better use of space and power when both are in limited supply. If you have two switches that each contain 24 ports, you would be able to support up to 46 devices, because you lose at least one port per switch to connect each switch to the rest of the network. In addition, two power outlets are required. On the other hand, if you have a single 48-port switch, 47 devices can be supported, with only one port used to connect the switch to the rest of the network, and only one power outlet needed to accommodate the single switch.
Modular switches can support very high port densities through the addition of multiple switch port line cards, as shown in the figure. For example, the Catalyst 6500 switch can support in excess of 1,000 switch ports on a single device.
Large enterprise networks that support many thousands of network devices require high density, modular switches to make the best use of space and power. Without using a high-density modular switch, the network would need many fixed configuration switches to accommodate the number of devices that need network access. This approach can consume many power outlets and a lot of closet space.
You must also address the issue of uplink bottlenecks. A series of fixed configuration switches may consume many additional ports for bandwidth aggregation between switches for the purpose of achieving target performance. With a single modular switch, bandwidth aggregation is less of an issue because the backplane of the chassis can provide the necessary bandwidth to accommodate the devices connected to the switch port line cards.
Forwarding rates define the processing capabilities of a switch by rating how much data the switch can process per second. Switch product lines are classified by forwarding rates. Entry-layer switches have lower forwarding rates than enterprise-layer switches. Forwarding rates are important to consider when selecting a switch. If the switch forwarding rate is too low, it cannot accommodate full wire-speed communication across all of its switch ports. Wire speed is the data rate that each port on the switch is capable of attaining, either 100 Mb/s Fast Ethernet or 1000 Mb/s Gigabit Ethernet. For example, a 48-port gigabit switch operating at full wire speed generates 48 Gb/s of traffic. If the switch only supports a forwarding rate of 32 Gb/s, it cannot run at full wire speed across all ports simultaneously. Fortunately, access layer switches typically do not need to operate at full wire speed because they are physically limited by their uplinks to the distribution layer. This allows you to use less expensive, lower performing switches at the access layer, and use the more expensive, higher performing switches at the distribution and core layers, where the forwarding rate makes a bigger difference.
As part of bandwidth aggregation, you should determine if there are enough ports on a switch to aggregate to support the required bandwidth. For example, consider a Gigabit Ethernet port, which carries up to 1 Gb/s of traffic. If you have a 24-port switch, with all ports capable of running at gigabit speeds, you could generate up to 24 Gb/s of network traffic. If the switch is connected to the rest of the network by a single network cable, it can only forward 1 Gb/s of the data to the rest of the network. Due to the contention for bandwidth, the data would forward more slowly. That results in 1/24th wire speed available to each of the 24 devices connected to the switch. Wire speed describes the theoretical maximum data transmission rate of a connection. For example, the wire speed of an Ethernet connection is dependent on the physical and electrical properties of the cable, combined with the lowest layer of the connection protocols.
Link aggregation helps to reduce these bottlenecks of traffic by allowing up to eight switch ports to be bound together for data communications, providing up to 8 Gb/s of data throughput when Gigabit Ethernet ports are used. With the addition of multiple 10 Gigabit Ethernet (10GbE) uplinks on some enterprise-layer switches, very high throughput rates can be achieved. Cisco uses the term EtherChannel when describing aggregated switch ports.
EtherChannel technology allows a group of physical Ethernet links to create one logical Ethernet link for the purpose of providing fault tolerance and high-speed links between switches, routers, and servers.
III. PoE and Layer 3 Functionality
Two other characteristics you want to consider when selecting a switch are Power over Ethernet (PoE) and Layer 3 functionality.
Power over Ethernet
Power over Ethernet (PoE) allows the switch to deliver power to a device over the existing Ethernet cabling. PoE allows you more flexibility when installing wireless access points and IP phones because you can install them anywhere you can run an Ethernet cable. You do not need to consider how to run ordinary power to the device. You should only select a switch that supports PoE if you are actually going to take advantage of the feature, because it adds considerable cost to the switch.
Figure 1-8: Power over Ethernet (PoE)
Layer 3 Functions
Typically, switches operate at Layer 2 of the OSI reference model where they deal primarily with the MAC addresses of devices connected to switch ports. Layer 3 switches offer advanced functionality that will be discussed in greater detail in the later chapters of this course. Layer 3 switches are also known as multilayer switches.
Figure 1-9: Layer 3 functionality
SWITCH FEATURES IN A HIERARCHICAL NETWORK
I. Access Layer Switch Features
Now that you know which factors to consider when choosing a switch, let us examine which features are required at each layer in a hierarchical network. You will then be able to match the switch specification with its ability to function as an access, distribution, or core layer switch.
Access layer switches facilitate the connection of end node devices to the network. For this reason, they need to support features such as port security, VLANs, Fast Ethernet/Gigabit Ethernet, PoE, and link aggregation.
Port security allows the switch to decide how many or what specific devices are allowed to connect to the switch. All Cisco switches support port layer security. Port security is applied at the access. Consequently, it is an important first line of defense for a network. You will learn about port security in Chapter 2.
VLANs are an important component of a converged network. Voice traffic is typically given a separate VLAN. In this way, voice traffic can be supported with more bandwidth, more redundant connections, and improved security. Access layer switches allow you to set the VLANs for the end node devices on your network.
Port speed is also a characteristic you need to consider for your access layer switches. Depending on the performance requirements for your network, you must choose between Fast Ethernet and Gigabit Ethernet switch ports. Fast Ethernet allows up to 100 Mb/s of traffic per switch port. Fast Ethernet is adequate for IP telephony and data traffic on most business networks, however, performance is slower than Gigabit Ethernet ports. Gigabit Ethernet allows up to 1000 Mb/s of traffic per switch port. Most modern devices, such as workstations, notebooks, and IP phones, support Gigabit Ethernet. This allows for much more efficient data transfers, enabling users to be more productive. Gigabit Ethernet does have a drawback-switches supporting Gigabit Ethernet are more expensive.
Another feature requirement for some access layer switches is PoE. PoE dramatically increases the overall price of the switch across all Cisco Catalyst switch product lines, so it should only be considered when voice convergence is required or wireless access points are being implemented, and power is difficult or expensive to run to the desired location.
Link aggregation is another feature that is common to most access layer switches. Link aggregation allows the switch to use multiple links simultaneously. Access layer switches take advantage of link aggregation when aggregating bandwidth up to distribution layer switches.
Because the uplink connection between the access layer switch and the distribution layer switch is typically the bottleneck in communication, the internal forwarding rate of access layer switches does not need to be as high as the link between the distribution and access layer switches. Characteristics such as the internal forwarding rate are less of a concern for access layer switches because they only handle traffic from the end devices and forward it to the distribution layer switches.
In a converged network supporting voice, video and data network traffic, access layer switches need to support QoS to maintain the prioritization of traffic. Cisco IP phones are types of equipment that are found at the access layer. When a Cisco IP phone is plugged into an access layer switch port configured to support voice traffic, that switch port tells the IP phone how to send its voice traffic. QoS needs to be enabled on access layer switches so that voice traffic the IP phone has priority over, for example, data traffic.
II. Distribution Layer Switch Features
Distribution layer switches have a very important role on the network. They collect the data from all the access layer switches and forward it to the core layer switches. As you will learn later in this course, traffic that is generated at Layer 2 on a switched network needs to be managed, or segmented into VLANs, so it does not needlessly consume bandwidth throughout the network. Distribution layer switches provides the inter-VLAN routing functions so that one VLAN can communicate with another on the network. This routing typically takes place at the distribution layer because distribution layer switches have higher processing capabilities than the access layer switches. Distribution layer switches alleviate the core switches from needing to perform that task since the core is busy handling the forwarding of very high volumes of traffic. Because inter-VLAN routing is performed at the distribution layer, the switches at this layer need to support Layer 3 functions.
Another reason why Layer 3 functionality is required for distribution layer switches is because of the advanced security policies that can be applied to network traffic. Access lists are used to control how traffic flows through the network. An Access Control List (ACL) allows the switch to prevent certain types of traffic and permit others. ACLs also allow you to control which network devices can communicate on the network. Using ACLs is processing-intensive because the switch needs to inspect every packet and see if it matches one of the ACL rules defined on the switch. This inspection is performed at the distribution layer, because the switches at this layer typically have the processing capability to handle the additional load, and it also simplifies the use of ACLs. Instead of using ACLs for every access layer switch in the network, they are defined on the fewer distribution layer switches, making management of the ACLs much easier.
Quality of Service
The distribution layer switches also need to support QoS to maintain the prioritization of traffic coming from the access layer switches that have implemented QoS. Priority policies ensure that audio and video communications are guaranteed adequate bandwidth to maintain an acceptable quality of service. To maintain the priority of the voice data throughout the network, all of the switches that forward voice data must support QoS; if not all of the network devices support QoS, the benefits of QoS will be reduced. This results in poor performance and quality for audio and video communications.
The distribution layer switches are under high demand on the network because of the functions that they provide. It is important that distribution switches support redundancy for adequate availability. Loss of a distribution layer switch could have significant impact on the rest of the network because all access layer traffic passes through the distribution layer switches. Distribution layer switches are typically implemented in pairs to ensure availability. It is also recommended that distribution layer switches support multiple, hot swappable power supplies. Having more than one power supply allows the switch to continue operating even if one of the power supplies failed during operation. Having hot swappable power supplies allows you to change a failed power supply while the switch is still running. This allows you to repair the failed component without impacting the functionality of the network.
Finally, distribution layer switches need to support link aggregation. Typically, access layer switches use multiple links to connect to a distribution layer switch to ensure adequate bandwidth to accommodate the traffic generated on the access layer, and provide fault tolerance in case a link is lost. Because distribution layer switches accept incoming traffic from multiple access layer switches, they need to be able to forward all of that traffic as fast as possible to the core layer switches. As a result, distribution layer switches also need high-bandwidth aggregated links back to the core layer switches. Newer distribution layer switches support aggregated 10 Gigabit Ethernet (10GbE) uplinks to the core layer switches.
III. Core Layer Switch Features
The core layer of a hierarchical topology is the high-speed backbone of the network and requires switches that can handle very high forwarding rates. The required forwarding rate is largely dependent on the number of devices participating in the network. You determine your necessary forwarding rate by conducting and examining various traffic flow reports and user communities analyses. Based on your results, you can identify an appropriate switch to support the network. Take care to evaluate your needs for the present and near future. If you choose an inadequate switch to run in the core of the network, you face potential bottleneck issues in the core, slowing down all communications on the network.
The core layer also needs to support link aggregation to ensure adequate bandwidth coming into the core from the distribution layer switches. Core layer switches should have support for aggregated 10GbE connections, which is currently the fastest available Ethernet connectivity option. This allows corresponding distribution layer switches to deliver traffic as efficiently as possible to the core.
The availability of the core layer is also critical, so you should build in as much redundancy as you can. Layer 3 redundancy typically has a faster convergence than Layer 2 redundancy in the event of hardware failure. Convergence in this context refers to the time it takes for the network to adapt to a change, not to be confused with a converged network that supports data, audio, and video communications. With that in mind, you want to ensure that your core layer switches support Layer 3 functions. A complete discussion on the implications of Layer 3 redundancy is beyond the scope of this course. It remains an open question about the need for Layer 2 redundancy in this context. Layer 2 redundancy is examined in Chapter 5 when we discuss the spanning tree protocol (STP). Also, look for core layer switches that support additional hardware redundancy features like redundant power supplies that can be swapped while the switch continues to operate. Because of the high workload carried by core layer switches, they tend to operate hotter than access or distribution layer switches, so they should have more sophisticated cooling options. Many true, core layer-capable switches have the ability to swap cooling fans without having to turn the switch off.
For example, it would be disruptive to shut down a core layer switch to change a power supply or a fan in the middle of the day when the network usage is at its highest. To perform a hardware replacement, you could expect to have at least a 5 minute network outage, and that is if you are very fast at performing the maintenance. In a more realistic situation, the switch could be down for 30 minutes or more, which most likely is not acceptable. With hot-swappable hardware, there is no downtime during switch maintenance.
QoS is an important part of the services provided by core layer switches. For example, service providers (who provide IP, data storage, e-mail and other services) and enterprise Wide Area Networks (WANs), are adding more voice and video traffic to an already growing amount of data traffic. At the core and network edge, mission-critical and time-sensitive traffic such as voice should receive higher QoS guarantees than less time-sensitive traffic such as file transfers or e-mail. Since high-speed WAN access is often prohibitively expensive, adding bandwidth at the core layer is not an option. Because QoS provides a software based solution to prioritize traffic, core layer switches can provide a cost effect way of supporting optimal and differentiated use of existing bandwidth.
SWITCHES FOR SMALL AND MEDIUM-SIZED BUSINESSES (SMB)
The features of Cisco Catalyst Switches
Now that you know which switch features are used at which layer in a hierarchical network, you will learn about the Cisco switches that are applicable for each layer in the hierarchical network model. Today, you cannot simply select a Cisco switch by considering the size of a business. A small business with 12 employees might be integrated into the network of a large multinational enterprise and require all of the advanced LAN services available at the corporate head office. The following classification of Cisco switches within the hierarchical network model represents a starting point for your deliberations on which switch is best for a given application. The classification presented reflects how you might see the range of Cisco switches if you were a multinational enterprise. For example, the port densities of the Cisco 6500 switch only makes sense as an access layer switch where there are many hundreds of users in one area, such as the floor of a stock exchange. If you think of the needs of a medium-sized business, a switch that is shown as an access layer switch, the Cisco 3560 for example, could be used as a distribution layer switch if it met the criteria determined by the network designer for that application.
Cisco has seven switch product lines. Each product line offers different characteristics and features, allowing you to find the right switch to meet the functional requirements of your network. The Cisco switch product lines are:
Catalyst Express 500
Figure 1-10: Features of Cisco Catalyst Switches
Catalyst Express 500
The Catalyst Express 500 is Cisco's entry-layer switch. It offers the following:
Forwarding rates from 8.8 Gb/s to 24 Gb/s
Layer 2 port security
Converged data/IP communications support
This switch series is appropriate for access layer implementations where high port density is not required. The Cisco Catalyst Express 500 series switches are scaled for small business environments ranging from 20 to 250 employees. The Catalyst Express 500 series switches are available in different fixed configurations:
Fast Ethernet and Gigabit Ethernet connectivity
Up to 24 10/100 ports with optional PoE or 12 10/100/1000 ports
Catalyst Express 500 series switches do not allow management through the Cisco IOS CLI. They are managed using a built-in web management interface, the Cisco Network Assistant or the new Cisco Configuration Manager developed specifically for the Catalyst Express 500 series switches. The Catalyst Express does not support console access.
To learn more about the Cisco Express 500 series of switches, go to http://www.cisco.com/en/US/products/ps6545/index.html.
The Catalyst 2960 series switches enable entry-layer enterprise, medium-sized, and branch office networks to provide enhanced LAN services. The Catalyst 2960 series switches are appropriate for access layer implementations where access to power and space is limited. The CCNA Exploration 3 LAN Switching and Wireless labs are based on the features of the Cisco 2960 switch.
The Catalyst 2960 series switches offers the following:
Forwarding rates from 16 Gb/s to 32 Gb/s
QoS features to support IP communications
Access control lists (ACLs)
Fast Ethernet and Gigabit Ethernet connectivity
Up to 48 10/100 ports or 10/100/1000 ports with additional dual purpose gigabit uplinks
The Catalyst 2960 series of switches do not support PoE.
The Catalyst 2960 series supports the Cisco IOS CLI, integrated web management interface, and Cisco Network Assistant. This switch series supports console and auxiliary access to the switch.
To learn more about the Catalyst 2960 series of switches, visit http://www.cisco.com/en/US/products/ps6406/index.html.
The Cisco Catalyst 3560 series is a line of enterprise-class switches that include support for PoE, QoS, and advanced security features such as ACLs. These switches are ideal access layer switches for small enterprise LAN access or branch-office converged network environments.
The Cisco Catalyst 3560 Series supports forwarding rates of 32 Gb/s to 128 Gb/s (Catalyst 3560-E switch series).
The Catalyst 3560 series switches are available in different fixed configurations:
Fast Ethernet and Gigabit Ethernet connectivity
Up to 48 10/100/1000 ports, plus four Small Form-Factor Pluggable (SFP) ports
Optional 10 Gigabit Ethernet connectivity in the Catalyst 3560-E models
Optional Integrated PoE (Cisco pre-standard and IEEE 802.3af); up to 24 ports with 15.4 watts or 48 ports with 7.3 watts
To learn more about the Catalyst 3560 series of switches, visit http://www.cisco.com/en/US/products/hw/switches/ps5528/index.html.
The Cisco Catalyst 3750 series of switches are ideal for access layer switches in midsize organizations and enterprise branch offices. This series offers forwarding rates from 32 Gb/s to 128 Gb/s (Catalyst 3750-E switch series). The Catalyst 3750 series supports Cisco StackWise technology. StackWise technology allows you to interconnect up to nine physical Catalyst 3750 switches into one logical switch using a high-performance (32 Gb/s), redundant, backplane connection.
The Catalyst 3750 series switches are available in different stackable fixed configurations:
Fast Ethernet and Gigabit Ethernet connectivity
Up to 48 10/100/1000 ports, plus four SFP ports
Optional 10 Gigabit Ethernet connectivity in the Catalyst 3750-E models
Optional Integrated PoE (Cisco pre-standard and IEEE 802.3af); up to 24 ports with 15.4 watts or 48 ports with 7.3 watts
To learn more about the Catalyst 3750 series of switches, visit http://www.cisco.com/en/US/products/hw/switches/ps5023/index.html.
The Catalyst 4500 is the first midrange modular switching platform offering multilayer switching for enterprises, small- to medium-sized businesses, and service providers.
With forwarding rates up to 136 Gb/s, the Catalyst 4500 series is capable of managing traffic at the distribution layer. The modular capability of the Catalyst 4500 series allows for very high port densities through the addition of switch port line cards to its modular chassis. The Catalyst 4500 series offers multilayer QoS and sophisticated routing functions.
The Catalyst 4500 series switches are available in different modular configurations:
Modular 3, 6, 7, and 10 slot chassis offering different layers of scalability
High port density: up to 384 Fast Ethernet or Gigabit Ethernet ports available in copper or fiber with 10 Gigabit uplinks
PoE (Cisco pre-standard and IEEE 802.3af)
Dual, hot-swappable internal AC or DC power supplies
Advanced hardware-assisted IP routing capabilities
To learn more about the Catalyst 4500 series of switches, visit http://www.cisco.com/en/US/products/hw/switches/ps4324/index.html.
The Catalyst 4900 series switches are designed and optimized for server switching by allowing very high forwarding rates. The Cisco Catalyst 4900 is not a typical access layer switch. It is a specialty access layer switch designed for data center deployments where many servers may exist in close proximity. This switch series supports dual, redundant power supplies and fans that can be swapped out while the switch is still running. This allows the switches to achieve higher availability, which is critical in data center deployments.
The Catalyst 4900 series switches support advanced QoS features, making them ideal candidates for the back-end IP telephony hardware. Catalyst 4900 series switches do not support the StackWise feature of the Catalyst 3750 series nor do they support PoE.
The Catalyst 4900 series switches are available in different fixed configurations:
Up to 48 10/100/1000 ports with four SFP ports or 48 10/100/1000 ports with two 10GbE ports
Dual, hot-swappable internal AC or DC power supplies
Hot-swappable fan trays
To learn more about the Catalyst 4900 series of switches, visit http://www.cisco.com/en/US/products/ps6021/index.html.
The Catalyst 6500 series modular switch is optimized for secure, converged voice, video, and data networks. The Catalyst 6500 is capable of managing traffic at the distribution and core layers. The Catalyst 6500 series is the highest performing Cisco switch, supporting forwarding rates up to 720 Gb/s. The Catalyst 6500 is ideal for very large network environments found in enterprises, medium-sized businesses, and service providers.
The Catalyst 6500 series switches are available in different modular configurations:
Modular 3, 4, 6, 9, and 13 slot chassis
LAN/WAN service modules
PoE up to 420 IEEE 802.3af Class 3 (15.4W) PoE devices
Up to 1152 10/100 ports, 577 10/100/1000 ports, 410 SFP Gigabit Ethernet ports, or 64 10 Gigabit Ethernet ports
Dual, hot-swappable internal AC or DC power supplies
Advanced hardware-assisted IP routing capabilities
To learn more about the Catalyst 6500 series of switches, visit http://www.cisco.com/en/US/products/hw/switches/ps708/index.html
The following tool can help identify the correct switch for an implementation: http://www.cisco.com/en/US/products/hw/switches/products_promotion0900aecd8050364f.html
The following guide provides a detailed comparison of current switch offerings from Cisco: http://www.cisco.com/application/pdf/en/us/guest/products/ps708/c2072/cdccont_0900aecd805f0955.pdf
1.3 CHAPTER SUMMARY
In this chapter, we discussed the hierarchical design model. Implementing this model improves the performance, scalability, availability, manageability, and maintainability of the network. Hierarchical network topologies facilitate network convergence by enhancing the performance necessary for voice and video data to be combined onto the existing data network.
Traffic flow, user communities, data stores and server location, and topology diagram analysis are used to help identify network bottlenecks. The bottlenecks can then be addressed to improve the performance of the network and accurately determine appropriate hardware requirements to satisfy the desired performance of the network.
We surveyed the different switch features, such as form factor, performance, PoE, and Layer 3 support and how they relate to the different layers of the hierarchical network design. An array of Cisco Catalyst switch product lines is available to support any application or business size.