Summary

The beginning of this chapter examines the fundamentals of network design and discusses basic design principles, such as hierarchy, scalability, and convergence. Addressing, summarization, and redistribution are noted as critical factors in designing a scalable network. Then, the chapter discusses and evaluates various design principles from the standpoint of IS-IS. The use of Integrated IS-IS as an IGP is covered in detail. This discussion considers the limitations and strengths of the IS-IS protocol and interaction with BGP.

Scaling issues are presented and the issues of network stability and convergence are discussed in relation to IS-IS. The size of a network is isolated as a key contributor to network performance. The effect of the number of nodes and number of links are discussed. The chapter also points out that stability of the links also contributes to the overall stability of the network. Various design options are discussed for IS-IS applications, including hierarchical designs with multiple areas, and flat single-area designs are also covered. Hierarchy is discussed as a good approach for constraining network instabilities and for network growth and expansion. The chapter defines design trade-offs between network stability and fast convergence and indicates that, overall, a fast route processor is a key asset for achieving fast convergence.

A key theme of the chapter is that design objectives and expectations should always be clearly stated as a philosopy behind any build out. The later sections of the chapter review the exponential back-off feature, which can be used to mitigate a good compromise between the conflicting design goals of fast convergence and stability. Exponential backoff allows for quick reponse to network changes while retaining the capability to slow down actions to contain massive instabilities during persistent changes in the network.

A considerable amount of space in the chapter is dedicated to comparative analysis of IS-IS versus OSPF. The comparison takes off with a brief historical review of the origins of both protocols. Some time is spent on similarities between the two protocols, and highlights of the differences between them is presented in table format. The rest of the chapter discusses in-depth both architectural and implementation differences. A final conclusion is drawn that the two protocols are similar, for the most part, in functionality and have proven themselves in real-world deployments and either can do the job. Some guidelines are provided to assist network operators in making the difficult choice between the two.