Home > Articles > Networking > Network Administration & Management

Structuring and Modularizing the Network with Cisco Enterprise Architecture

This chapter introduces a modular hierarchical approach to network design, the Cisco Enterprise Architecture.
This chapter is from the book

This chapter includes the following sections:

  • Network Hierarchy
  • Using a Modular Approach to Network Design
  • Services Within Modular Networks
  • Network Management Protocols and Features
  • Summary
  • References
  • Case Study: ACMC Hospital Modularity
  • Review Questions

This chapter introduces a modular hierarchical approach to network design, the Cisco Enterprise Architecture. The chapter begins with a discussion of the hierarchical network structure. The next section introduces network modularization and discusses the details of the Cisco Enterprise Architecture. Following that are a detailed description of services within modular networks, and a discussion of network management protocols and features.

Network Hierarchy

This section explains the hierarchical network model, which is composed of the access, distribution, and core layers. The functions generally associated with each of these layers are discussed, as is the most common approach to designing a hierarchical network.

Historically used in the design of enterprise local-area network and wide-area network data networks, this model works equally well within the functional modules of the Cisco Enterprise Architecture. These modules are discussed later in this chapter, in the section "Using a Modular Approach to Network Design."

Hierarchical Network Model

The hierarchical network model provides a framework that network designers can use to help ensure that the network is flexible and easy to implement and troubleshoot.

Hierarchical Network Design Layers

As shown in Figure 3-1, the hierarchical network design model consists of three layers:

  • The access layer provides local and remote workgroup or user access to the network.
  • The distribution layer provides policy-based connectivity.
  • The core (or backbone) layer provides high-speed transport to satisfy the connectivity and transport needs of the distribution layer devices.
Figure 3-1

Figure 3-1 Hierarchical Model's Three Layers

Each hierarchical layer focuses on specific functions, thereby allowing the network designer to choose the right systems and features based on their function within the model. This approach helps provide more accurate capacity planning and minimize total costs. Figure 3-2 illustrates a sample network showing the mapping to the hierarchical model's three layers.

Figure 3-2

Figure 3-2 Sample Network Designed Using the Hierarchical Model

You do not have to implement the hierarchical layers as distinct physical entities; they are defined to aid successful network design and to represent functionality that must exist within a network. The actual manner in which you implement the layers depends on the needs of the network you are designing. Each layer can be implemented in routers or switches, represented by physical media, or combined in a single device. A particular layer can be omitted, but hierarchy should be maintained for optimum performance. The following sections detail the functionality of the three layers and the devices used to implement them.

Access Layer Functionality

This section describes the access layer functions and the interaction of the access layer with the distribution layer and local or remote users.

The Role of the Access Layer

The access layer is the concentration point at which clients access the network. Access layer devices control traffic by localizing service requests to the access media.

The purpose of the access layer is to grant user access to network resources. Following are the access layer's characteristics:

  • In the campus environment, the access layer typically incorporates switched LAN devices with ports that provide connectivity for workstations and servers.
  • In the WAN environment, the access layer for teleworkers or remote sites provides access to the corporate network across some wide-area technology, such as Frame Relay, Multiprotocol Label Switching (MPLS), Integrated Services Digital Network, leased lines, Digital Subscriber Line (DSL) over traditional telephone copper lines, or coaxial cable.
  • So as not to compromise network integrity, access is granted only to authenticated users or devices (such as those with physical address or logical name authentication). For example, the devices at the access layer must detect whether a telecommuter who is dialing in is legitimate, yet they must require minimal authentication steps for the telecommuter.

Layer 2 and Multilayer Switching in the Access Layer

Access can be provided to end users as part of either a Layer 2 (L2) switching environment or a multilayer switching environment.

Using Layer 2 Switching in the Access Layer

Access to local workstations and servers can be provided using shared or switched media LANs; VLANs may be used to segment the switched LANs. Each LAN or VLAN is a single broadcast domain.

The access layer aggregates end-user switched 10/100 ports and provides Fast Ethernet, Fast EtherChannel, and Gigabit Ethernet uplinks to the distribution layer to satisfy connectivity requirements and reduce the size of the broadcast domains. You can deploy multiple VLANs, each with its own IP subnet and its own instance of Spanning Tree Protocol (STP) providing alternative paths in case of failure. In this case, Layer 2 trunking (typically using the Institute for Electrical and Electronic Engineers [IEEE] 802.1Q trunking protocol) is used between the access layer switches and the distribution layer switches, with per-VLAN STP on each uplink for load balancing and redundancy, and with a distribution layer multilayer switch providing the inter-VLAN communication for the access layer.

When RSTP cannot be implemented, Cisco IOS STP features such as UplinkFast, PortFast, and BackboneFast can be used to provide equivalent convergence improvements. These features are described as follows:

  • UplinkFast: Enables faster failover on an access layer switch on which dual uplinks connect to the distribution layer. The failover time is reduced by unblocking the blocked uplink port on a switch immediately after root port failure, thereby transitioning it to the forwarding state immediately, without transitioning the port through the listening and learning states.
  • BackboneFast: If a link fails on the way to the root switch but is not directly connected to the local switch, BackboneFast reduces the convergence time from 50 seconds to between 20 and 30 seconds.
  • PortFast: Enables switch ports connected to nonswitch devices (such as workstations) to immediately enter the spanning-tree forwarding state, thereby bypassing the listening and learning states, when they come up. Ports connected only to an end-user device do not have bridging loops, so it is safe to go directly to the forwarding state, significantly reducing the time it takes before the port is usable.
Using Multilayer Switching in the Access Layer

The most common design for remote users is to use multilayer switches or routers. A multilayer switch, or router, is the boundary for broadcast domains and is necessary for communicating between broadcast domains (including VLANs). Access routers provide access to remote office environments using various wide-area technologies combined with multilayer features, such as route propagation, packet filtering, authentication, security, Quality of Service (QoS), and so on. These technologies allow the network to be optimized to satisfy a particular user's needs. In a dialup connection environment, dial-on-demand routing (DDR) and static routing can be used to control costs.

Access Layer Example

Figure 3-3 illustrates a sample network in which the campus access layer aggregates end users and provides uplinks to the distribution layer. The access layer switches are dual-attached to the distribution layer switches for high availability.

Figure 3-3

Figure 3-3 Access Layer Connectivity in a Campus LAN

The access layer can support convergence, high availability, security, QoS, and IP multicast. Some services found at the access layer include establishing a QoS trust boundary, broadcast suppression, and Internet Group Management Protocol (IGMP) snooping.

Distribution Layer Functionality

This section describes distribution layer functions and the interaction of the distribution layer with the core and access layers.

The Role of the Distribution Layer

The distribution layer represents both a separation between the access and core layers and a connection point between the diverse access sites and the core layer. The distribution layer determines department or workgroup access and provides policy-based connectivity.

Following are the characteristics of the distribution layer:

  • Distribution layer devices control access to resources that are available at the core layer and must therefore use bandwidth efficiently.
  • In a campus environment, the distribution layer aggregates wiring closet bandwidth by concentrating multiple low-speed access links into a high-speed core link and using switches to segment workgroups and isolate network problems to prevent them from affecting the core layer.
  • Similarly, in a WAN environment, the distribution layer aggregates WAN connections at the edge of the campus and provides policy-based connectivity.
  • This layer provides redundant connections for access devices. Redundant connections also provide the opportunity to load-balance between devices.
  • The distribution layer represents a routing boundary between the access and core layers and is where routing and packet manipulation are performed.
  • The distribution layer allows the core layer to connect diverse sites while maintaining high performance. To maintain good performance in the core, the distribution layer can redistribute between bandwidth-intensive access-layer routing protocols and optimized core routing protocols. Route filtering is also implemented at the distribution layer.
  • The distribution layer can summarize routes from the access layer to improve routing protocol performance. For some networks, the distribution layer offers a default route to access-layer routers and runs dynamic routing protocols only when communicating with core routers.
  • The distribution layer connects network services to the access layer and implements policies for QoS, security, traffic loading, and routing. For example, the distribution layer addresses different protocols' QoS needs by implementing policy-based traffic control to isolate backbone and local environments. Policy-based traffic control prioritizes traffic to ensure the best performance for the most time-critical and time-dependent applications.
  • The distribution layer is often the layer that terminates access layer VLANs (broadcast domains); however, this can also be done at the access layer.
  • This layer provides any media transitions (for example, between Ethernet and ATM) that must occur.

Distribution Layer Example

Figure 3-4 shows a sample network with various features of the distribution layer highlighted.

Figure 3-4

Figure 3-4 Example of Distribution Layer Features

Following are the characteristics of the distribution layer in the routed campus network shown in Figure 3-4:

  • Multilayer switching is used toward the access layer (and, in this case, within the access layer).
  • Multilayer switching is performed in the distribution layer and extended toward the core layer.
  • The distribution layer performs two-way route redistribution to exchange the routes between the Routing Information Protocol version 2 (RIPv2) and Enhanced Interior Gateway Routing Protocol (EIGRP) routing processes.
  • Route filtering is configured on the interfaces toward the access layer.
  • Route summarization is configured on the interfaces toward the core layer.
  • The distribution layer contains highly redundant connectivity, both toward the access layer and toward the core layer.

Core Layer Functionality

This section describes core layer functions and the interaction of the core layer with the distribution layer.

The Role of the Core Layer

The function of the core layer is to provide fast and efficient data transport. Characteristics of the core layer include the following:

  • The core layer is a high-speed backbone that should be designed to switch packets as quickly as possible to optimize communication transport within the network.
  • Because the core is critical for connectivity, core layer devices are expected to provide a high level of availability and reliability. A fault-tolerant network design ensures that failures do not have a major impact on network connectivity. The core must be able to accommodate failures by rerouting traffic and responding quickly to changes in network topology. The core must provide a high level of redundancy. A full mesh is strongly suggested, and at least a well-connected partial mesh with multiple paths from each device is required.
  • The core layer should not perform any packet manipulation, such as checking access lists or filtering, which would slow down the switching of packets.
  • The core layer must be manageable.
  • The core devices must be able to implement scalable protocols and technologies, and provide alternative paths and load balancing.

Switching in the Core Layer

Layer 2 switching or multilayer switching (routing) can be used in the core layer. Because core devices are responsible for accommodating failures by rerouting traffic and responding quickly to network topology changes, and because performance for routing in the core with a multilayer switch incurs no cost, most implementations have multilayer switching in the core layer. The core layer can then more readily implement scalable protocols and technologies, and provide alternate paths and load balancing.

Figure 3-5 shows an example of Layer 2 switching in the campus core.

Figure 3-5

Figure 3-5 Layer 2 Switching in the Campus Core

In Figure 3-5, a typical packet between access sites follows these steps:

  • Step 1 The packet is Layer 2–switched toward a distribution switch.
  • Step 2 The distribution switch performs multilayer switching toward a core interface.
  • Step 3 The packet is Layer 2–switched across the LAN core.
  • Step 4 The receiving distribution switch performs multilayer switching toward an access layer LAN.
  • Step 5 The packet is Layer 2–switched across the access layer LAN to the destination host.

Figure 3-6 shows an example of multilayer switching in the campus core.

Figure 3-6

Figure 3-6 Multilayer Switching in the Campus Core

In Figure 3-6, a typical packet between access sites follows these steps:

  • Step 1 The packet is Layer 2–switched toward a distribution switch.
  • Step 2 The distribution switch performs multilayer switching toward a core interface.
  • Step 3 The packet is multilayer-switched across the LAN core.
  • Step 4 The receiving distribution switch performs multilayer switching toward an access LAN.
  • Step 5 The packet is Layer 2–switched across the access layer LAN to the destination host.

Hierarchical Routing in the WAN

Figure 3-7 shows an example of hierarchical routing in the WAN portion of a network.

Figure 3-7

Figure 3-7 Hierarchical Routing in the WAN

In Figure 3-7, a typical packet between access sites follows these steps:

  • Step 1 The packet is Layer 3–forwarded toward the distribution router.
  • Step 2 The distribution router forwards the packet toward a core interface.
  • Step 3 The packet is forwarded across the WAN core.
  • Step 4 The receiving distribution router forwards the packet toward the appropriate access layer router.
  • Step 5 The packet is Layer 3–forwarded to the destination host's access layer LAN.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020