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Cisco Networking Academy's Introduction to Switched Networks

Chapter Description

This chapter begins an examination of the flow of traffic in a modern network. It examines some of the current network design models and the way LAN switches build forwarding tables and use the MAC address information to efficiently switch data between hosts.

LAN Design (1.1)

Hiring managers want networking professionals, even entry level ones, to be able to design a LAN. Why is this so important? If someone knows how to design something, it means that person knows and understands the components that comprise the object. By knowing how to design a LAN, a network professional knows the network components and how those components interact with one another. The professional would also know what products to buy to expand the network.

Converged Networks (1.1.1)

The words converged network can mean several things to a network engineer: (1) a single network designed to handle voice, video, and data; (2) an internal network where the Layer 3 devices, such as routers, have a complete routing table to be able to accurately and efficiently send data to a remote destination; and (3) a switch network that has completed calculations that result in a single path through the switch network. In this chapter, we explore the first description.

Growing Complexity of Networks (

Our digital world is changing. The ability to access the Internet and the corporate network is no longer confined to physical offices, geographic locations, or time zones. In today’s globalized workplace, employees can access resources from anywhere in the world and information must be available at any time, and on any device. These requirements drive the need to build next-generation networks that are secure, reliable, and highly available.

These next generation networks must not only support current expectations and equipment, but must also be able to integrate legacy platforms. Figure 1-1 shows some common legacy devices that must often be incorporated into network design. Figure 1-2 illustrates some of the newer platforms (converged networks) that help to provide access to the network anytime, anywhere, and on any device.

Figure 1-1

Figure 1-1 Legacy Components

Figure 1-2

Figure 1-2 Converged Network Components

Elements of a Converged Network (

To support collaboration, business networks employ converged solutions using voice systems, IP phones, voice gateways, video support, and video conferencing (Figure 1-3). Including data services, a converged network with collaboration support may include features such as the following:

  • Call control: Telephone call processing, caller ID, call transfer, hold, and conference
  • Voice messaging: Voicemail
  • Mobility: Receive important calls wherever you are
  • Automated attendant: Serve customers faster by routing calls directly to the right department or individual

    Figure 1-3

    Figure 1-3 Network Traffic Convergence

One of the primary benefits of transitioning to the converged network is that there is just one physical network to install and manage. This results in substantial savings over the installation and management of separate voice, video, and data networks. Such a converged network solution integrates IT management so that any moves, additions, and changes are completed with an intuitive management interface. A converged network solution also provides PC softphone application support, as well as point-to-point video so that users can enjoy personal communications with the same ease of administration and use as a voice call.

The convergence of services onto the network has resulted in an evolution in networks from a traditional data transport role, to a super-highway for data, voice, and video communication. This one physical network must be properly designed and implemented to allow the reliable handling of the various types of information that it must carry. A structured design is required to allow management of this complex environment.

Play the online video to view a few of the collaboration services in action.

Borderless Switched Networks (

With the increasing demands of the converged network, the network must be developed with an architectural approach that embeds intelligence, simplifies operations, and is scalable to meet future demands. One of the more recent developments in network design is illustrated by the Cisco Borderless Network architecture illustrated Figure 1-4.

Figure 1-4

Figure 1-4 Borderless Switched Networks

The Cisco Borderless Network is a network architecture that combines several innovations and design considerations to allow organizations to connect anyone, anywhere, anytime, and on any device securely, reliably, and seamlessly. This architecture is designed to address IT and business challenges, such as supporting the converged network and changing work patterns.

The Cisco Borderless Network is built on an infrastructure of scalable and resilient hardware and software. It enables different elements, from access switches to wireless access points, to work together and allow users to access resources from any place at any time, providing optimization, scalability, and security to collaboration and virtualization.

Play the online video to learn more about the evolution of the Cisco Borderless Network.

Hierarchy in the Borderless Switched Network (

Creating a borderless switched network requires that sound network design principles are used to ensure maximum availability, flexibility, security, and manageability. The borderless switched network must deliver on current requirements and future required services and technologies. Borderless switched network design guidelines are built upon the following principles:

  • Hierarchical: Facilitates understanding the role of each device at every tier, simplifies deployment, operation, and management, and reduces fault domains at every tier
  • Modularity: Allows seamless network expansion and integrated service enablement on an on-demand basis
  • Resiliency: Satisfies user expectations for keeping the network always on
  • Flexibility: Allows intelligent traffic load sharing by using all network resources

These are not independent principles. Understanding how each principle fits in the context of the others is critical. Designing a borderless switched network in a hierarchical fashion creates a foundation that allows network designers to overlay security, mobility, and unified communication features. Two time-tested and proven hierarchical design frameworks for campus networks are the three-tier layer and the two-tier layer models, as illustrated in Figure 1-5.

Figure 1-5

Figure 1-5 Switch Network Design Models

The three critical layers within these tiered designs are the access, distribution, and core layers. Each layer can be seen as a well-defined, structured module with specific roles and functions in the campus network. Introducing modularity into the campus hierarchical design further ensures that the campus network remains resilient and flexible enough to provide critical network services. Modularity also helps to allow for growth and changes that occur over time.

Core Distribution Access (

There are three layers of distribution access:

  • Access layer
  • Distribution layer
  • Core layer

These will be discussed in greater detail in this section.

Access Layer

The access layer represents the network edge, where traffic enters or exits the campus network. Traditionally, the primary function of an access layer switch is to provide network access to the user. Access layer switches connect to distribution layer switches, which implement network foundation technologies such as routing, quality of service, and security.

To meet network application and end-user demand, the next-generation switching platforms now provide more converged, integrated, and intelligent services to various types of endpoints at the network edge. Building intelligence into access layer switches allows applications to operate on the network more efficiently and securely.

Distribution Layer

The distribution layer interfaces between the access layer and the core layer to provide many important functions, including:

  • Aggregating large-scale wiring closet networks
  • Aggregating Layer 2 broadcast domains and Layer 3 routing boundaries
  • Providing intelligent switching, routing, and network access policy functions to access the rest of the network
  • Providing high availability through redundant distribution layer switches to the end-user and equal cost paths to the core
  • Providing differentiated services to various classes of service applications at the edge of the network
Core Layer

The core layer is the network backbone. It connects several layers of the campus network. The core layer serves as the aggregator for all of the other campus blocks and ties the campus together with the rest of the network. The primary purpose of the core layer is to provide fault isolation and high-speed backbone connectivity.

Figure 1-6 shows a three-tier campus network design for organizations where the access, distribution, and core are each separate layers. To build a simplified, scalable, cost-effective, and efficient physical cable layout design, the recommendation is to build an extended-star physical network topology from a centralized building location to all other buildings on the same campus.

Figure 1-6

Figure 1-6 Three-Tier Campus Network Design

In some cases, because of a lack of physical or network scalability restrictions, maintaining a separate distribution and core layer is not required. In smaller campus locations where there are fewer users accessing the network or in campus sites consisting of a single building, separate core and distribution layers may not be needed. In this scenario, the recommendation is the alternate two-tier campus network design, also known as the collapsed core network design.

Figure 1-7 shows a two-tier campus network design example for an enterprise campus where the distribution and core layers are collapsed into a single layer.

Figure 1-7

Figure 1-7 Two-Tier Campus Network Design

Switched Networks (1.1.2)

Switched networks are important when deploying wired LANs. A network professional today must be well-versed in switches and LAN technology in order to add commonly deployed devices such as PCs, printers, video cameras, phones, copiers, and scanners. Sharing and accessing network devices is common in both the home and business network.

Role of Switched Networks (

The role of switched networks has evolved dramatically in the last two decades. It was not long ago that flat Layer 2 switched networks were the norm. Flat Layer 2 data networks relied on the basic properties of Ethernet and the widespread use of hub repeaters to propagate LAN traffic throughout an organization. As shown in Figure 1-8, networks have fundamentally changed to switched LANs in a hierarchical network. A switched LAN allows more flexibility, traffic management, and additional features, such as:

  • Quality of service
  • Additional security
  • Support for wireless networking and connectivity
  • Support for new technologies, such as IP telephony and mobility services

    Figure 1-8

    Figure 1-8 Hierarchical Networks

Figure 1-9 shows the hierarchical design used in the borderless switched network.

Figure 1-9

Figure 1-9 Three-Tier Design in Borderless Switched Networks

Form Factors (

There are various types of switches used in business networks. It is important to deploy the appropriate types of switches based on network requirements. Table 1-1 highlights some common business considerations when selecting switch equipment.

Table 1-1 Business Considerations for Switch Selection

Switch Feature

Business Consideration


The cost of a switch will depend on the number and speed of the interfaces, supported features, and expansion capability.

Port density

Network switches must support the appropriate number of devices on the network.


It is now common to power access points, IP phones, and even compact switches using Power over Ethernet (PoE). In addition to PoE considerations, some chassis-based switches support redundant power supplies.


The switch should provide continuous access to the network.

Port speed

The speed of the network connection is of primary concern to the end users.

Frame buffers

The capability of the switch to store frames is important in a network where there may be congested ports to servers or other areas of the network.


The number of users on a network typically grows over the switch should provide the opportunity for growth.

When selecting the type of switch, the network designer must choose between a fixed or a modular configuration, and stackable or non-stackable. Another consideration is the thickness of the switch, which is expressed in number of rack units. This is important for switches that are mounted in a rack. For example, the fixed configuration switches shown in Figure 1-10 are all 1 rack unit (1U). These options are sometimes referred to as switch form factors.

Figure 1-10

Figure 1-10 Fixed Configuration Switches

Fixed Configuration Switches

Fixed configuration switches do not support features or options beyond those that originally came with the switch (refer to Figure 1-10). The particular model determines the features and options available. For example, a 24-port gigabit fixed switch cannot support additional ports. There are typically different configuration choices that vary in how many and what types of ports are included with a fixed configuration switch.

Modular Configuration Switches

Modular configuration switches offer more flexibility in their configuration. Modular configuration switches typically come with different sized chassis that allow for the installation of different numbers of modular line cards (Figure 1-11). The line cards actually contain the ports. The line card fits into the switch chassis the way that expansion cards fit into a PC. The larger the chassis, the more modules it can support. There can be many different chassis sizes to choose from. A modular switch with a 24-port line card supports an additional 24-port line card, to bring the total number of ports up to 48.

Figure 1-11

Figure 1-11 Modular Configuration Switches

Stackable Configuration Switches

Stackable configuration switches can be interconnected using a special cable that provides high-bandwidth throughput between the switches (Figure 1-12). Cisco StackWise technology allows the interconnection of up to nine switches. Switches can be stacked one on top of the other with cables connecting the switches in a 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. Many Cisco stackable switches also support StackPower technology, which enables power sharing among stack members.

Figure 1-12

Figure 1-12 Stackable Configuration Switches

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