Advanced OSPF Design Concepts

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OSPF Implementation, Troubleshooting & Management

“The Man in the Arena: It is not the critic who counts, not the one who points out how the strong man stumbled or how the doer of deeds might have done them better. The credit belongs to the man who is actually in the arena, whose face is marred with sweat, dust, and blood; who strives valiantly; who errs and comes short again and again; who knows the great enthusiasm, the great devotions, and spends himself in a worthy cause; who if he fails, at least while daring greatly, so that his place shall never be with those cold and timid souls who know neither victory nor defeat.”—Anonymous

This section moves into the actual design, implementation, and management of an OSPF network. This information will be presented in three chapters:

Chapter 7, “Designing & Implementing an OSPF Network,” covers the actual process of sitting down and designing your OSPF network, including how to configure OSPF on Cisco routers.

Chapter 8, “Monitoring & Troubleshooting an OSPF Network,” covers how to go about monitoring and ensuring that OSPF is operating correctly and what to do if it is not.

Chapter 9, “Managing Your OSPF Network,” covers the fundamentals of OSPF network management as well as Simple Network Management Protocol (SNMP) and Management Information Bases (MIBs).

Chapter 7
Designing & Implementing an OSPF Network

“Imagination: A mind once stretched by a new idea never regains its original dimensions.”—Successories

This chapter covers the actual process of sitting down and designing your OSPF network. The real process of putting the pen to paper and the true process behind it is covered. It is this chapter’s intention to take the mystery out of designing any type of network. The concepts and steps discussed have universal application whether your network is BGP or OSPF; of course, the latter is emphasized. Chapter 6, “Advanced OSPF Design Concepts,” covered many of the commands necessary for configuring OSPF. In this chapter, you will become familiar with the necessary steps to actually begin the OSPF process on a Cisco router. You already know there are many potential network architectures where you would have to configure OSPF, and the most common are covered in this chapter. This chapter has two specific sections as follows:

  OSPF Network Design. This section reviews the specific network design goals that should be the general basis of every network. There are certain issues that you must be aware of as network designers, and they are discussed in this section. The six fundamental steps that make up the Network Design Methodology are covered with special enhancements given to issues regarding OSPF.
  Configuring OSPF on Cisco Routers. At this point in the book, you have everything you need to know about how OSPF works and how to go about designing and implementing an OSPF network. But how do you turn on OSPF? This section addresses basic and advanced configuration issues as relate to Cisco routers. A bonus area is covered as well that deals with multi-protocol routers.

OSPF Network Design

This book has discussed the various design techniques for OSPF, from the various golden rules to the number of routers per area. It is now time to actually take this information and begin the process of designing an OSPF network. Let’s begin the process by determining what is actually supported by Cisco Systems.

Cisco’s Implementation of OSPF

As discussed in Chapter 4, “Introduction to OSPF,” there is a variety of RFCs that deal with OSPF. By now, you should be familiar with the many different features available within the OSPF protocol. But which RFCs does Cisco support within its products?

  RFC 1253: Open Shortest Path First (OSPF) MIB. This RFC contains the information, which provides management information relating to OSPF.
  RFC 1583: OSPF Version 2. Cisco’s implementation conforms to the specifications as detailed in this RFC. They support the following key features: stub areas, route redistribution, authentication (covered later), tunable interface parameters, and virtual links.
  RFC 1587: Not-So-Stubby-Areas (NSSA). Cisco equipment supports the use of all types of stub areas.
  RFC 1793: OSPF over Demand Circuits. Cisco supports this RFC as well.

Network Design Goals

It is not necessary to get into the reasons behind your decision to build an OSPF network or any of the previously covered definitions of what a network is. However, the five basic goals that you should keep in mind while designing your OSPF network (or any network for that matter) should be adhered to:

  Cost effectiveness


“The network must work” is the absolute bottom line. Because networks are an integral part of enabling individual users to do their jobs, this is essential. It is here that the use of Service Level Agreements (SLAs) is essential. You must know what is expected of the network in order to design it properly.


As your organization grows, the network must be able to keep pace. Your network and its initial design must enable it to expand accordingly. A network that cannot keep pace with the organization’s needs is not much use.

Routing summarization is a major factor in the success of designing your network. If you want to ensure your network can scale properly, the summarization is the biggest factor on your success. Without summarization, you will have a flat address design with specific route information for every host being transmitted across the network, a very bad thing in large networks. To briefly review summarization, remember that routers summarize at several levels, as shown in Figure 7-1. For example, hosts are grouped into subnetworks, subnetworks are then grouped into major networks, and these are then consolidated in areas. The network can then be grouped into an autonomous system.

Figure 7-1  Route summarization affects network scalability.

There are many smaller networks that desire to use a “standard” routing protocol such as OSPF. These networks can, for example, have 100 or less routers with a relatively small IP space. In these situations, summarization may not be possible and might not gain much if it were implemented.

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OSPF Network Design Solutions
OSPF Network Design Solutions
ISBN: 1578700469
EAN: 2147483647
Year: 1998
Pages: 200
Authors: Tom Thomas

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