Home > Articles > Cisco Certification > CCIE > Cisco Network Topology and Design

Cisco Network Topology and Design

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Feb 1, 2002.

Chapter Description

Explore design issues related to overall network topology with this sample chapter from CCIE Professional Development: Large-Scale IP Network Solutions by Cisco Press.

Hierarchy Issues

Just as the Internet hierarchy had to adapt to accommodate an increase in players, so has the enterprise network. Early data networks facilitated the basic requirement of communication between terminals and a central mainframe. The need for a simple hierarchy, from terminal to IBM cluster controllers to front-end processors, was readily apparent from this basic requirement. In today's world of peer-to-peer networking, however, the reasons for network hierarchy and its inner workings are subtler, yet just as important for successful network design.

Figure 4-2 presents the high-level network architecture developed in Chapter 1. Notice that the network backbone (also called the network core—the terms core and backbone are equivalent) consists of mesh-connected backbone routers that reside at distribution centers (DCs) within each service region.

Each DC, which may house a LAN topology that is resilient to single node failure, forms the hub of a star distribution network for that region. Finally, the access network consists of both provider and customer premise equipment, which is typically homed to one or more access POPs.

Figure 4-2 Modularization of a Large Network

In the example in Figure 4-2, only three regions/DCs and, at most, three POPs in a region are shown. However, the hierarchical model will scale much more than this using current commercial routers.

A typical large IP network, whether an ISP or a large corporate intranet, will consist of routers performing a number of different roles. It is convenient to define three major roles corresponding to each layer in the hierarchy: backbone, distribution, and access.

As shown in Figure 4-3, which illustrates the arrangement in a typical high-resilience DC, these roles possess a specific hierarchical relationship. Backbone routers are at the top, distribution routers are in the middle, and access routers are at the bottom.

Figure 4-3 Distribution Center Architecture

Backbone Routers

The backbone routers core1.sfo and core2.sfo reside in the San Francisco DC and are responsible for connecting the regional network to the backbone. These routers forward packets to and from the region. They also advertise reachability for that region, either to the core routers of other regions (in other major cities), or to external peer networks (other ISPs).

Backbone routers are also peers in terms of the useful reachability information they possess. This does not imply that router core1.sfo has the same detailed topological information about Los Angeles as, say, router core1.lax, but it does indicate that core1.sfo understands that core1.lax and core2.lax, rather than core1.stl, are the gateways to all destinations in the Los Angeles region.

Backbone routers contain reachability intelligence for all destinations within the network. They possess the capability to distinguish between the gateway information and the information that explains how to reach the outside world, which is through other peer networks or the Internet.

Distribution Routers

Distribution routers consolidate connections from access routers. They are often arranged in a configuration that is resilient to failure of a single core router. Distribution routers usually contain topological information about their own region, but they forward packets to a backbone router for inter-region routing.


In smaller regions, distribution and backbone routers may be one and the same. In larger regions, distribution routers themselves may form a hierarchy.

High-performance customers on permanent WAN links often may connect directly to distribution routers, whereas dial-on-demand customers typically do not because this would impose the need to run dial-authentication software images of distribution routers.

Access Routers

Access routers connect the customer or enterprise site to the distribution network. In the ISP case, the router at the remote end of an access link is typically the customer premises equipment, and may be owned and operated by the customer.

For large enterprise networks, in which the LANs and WANs are managed by different divisions or contractors, the access router typically is managed by either the WAN or the LAN operator—usually this is the latter if the LAN is very large.

You now may wonder: Why is it important to distinguish between the backbone, access, and distribution routers? The reason is that they are increasingly becoming very distinct hardware/software combinations. In access routers, for example, you already have seen the need for support of dial-on-demand and authentication, as well as route filtering and packet filtering and classification.

In distribution routers, the emphasis is on economical aggregation of traffic and the support of varied media WAN types and protocols. In backbone routers, the emphasis is on supporting extremely high speeds, and aggregation of a very limited set of media types and routing protocols. These differences are summarized in Table 4-1.

Table 4-1 Characteristics of Backbone, Distribution, and Access Routers

Router Type


Backbone router

Scalable: packet forwarding, WAN links, QoS, routing Expensive Redundant WAN links National infrastructure

Distribution router

Scalable: WAN aggregation, LAN speeds Redundant LAN links Less expensive

Access router

Scalable: WAN aggregation Cheap Complex routing/QoS policy setting, access security, and monitoring capabilities

This discussion focused attention on the WAN environment and has avoided any issues of LAN design, other than the use of specific LAN technology within the distribution or access networks. In particular, at the individual user or network host level, access technologies include ATM, FDDI, Token Ring, or the ubiquitous Ethernet; rather than such technologies as Frame Relay, T1, SMDS, and SONET.

Scaling LANs through the use of hierarchy is itself the subject of much literature. To study this area further, interested readers should refer to the references listed at the end of this chapter.

The origins of the three-tiered, backbone-distribution-access hierarchy can be traced to the evolution of the Internet (refer to Chapter 1). However, hierarchical design is certainly nothing new and has been used in telephone networks and other systems for many years. In the case of IP data networking, there are several reasons for adding hierarchy.

Not only does hierarchy allow the various elements of routing, QoS, accounting, and packet switching to scale; but it also presents the opportunity for operational segmentation of the network, simpler troubleshooting, less complicated individual router configurations, and a logical basis for distance-based packet accounting.

These issues are examined in great depth in Part II of this book, "Core and Distributing Networks." For the moment, we will examine the topologies used within the backbone, distribution, and access layers of the network architecture.

4. Backbone Core Network Design | Next Section Previous Section

Cisco Press Promotional Mailings & Special Offers

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


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Cisco Press 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.


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.


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


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


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


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.


Users can always make an informed choice as to whether they should proceed with certain services offered by Cisco Press. 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.ciscopress.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.


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