Chapter 7. General Network Design Issues

Significant issues that deserve consideration in general network design mostly relate to scalability, stability, and convergence. This applies to the design of the physical layout of the network, as well as routing and traffic management. Scalability, stability, and convergence are relevant to both interdomain and intradomain routing. For intradomain routing, these issues deserve careful consideration, regardless of the IGP selected; and in all cases, only a systematic, well-structured approach will lead to the desired objectives of the design.

The following elements of network design directly impact network scalability, stability, and convergence and are certainly important considerations when designing Integrated IS-IS networks:

• Hierarchy ” The network architect must decide to deploy IS-IS in flat architecture or with a two-level hierarchy. This also means deciding whether to run the whole domain logically flat (Level 1-only or Level 2-only) or to use hierarchy with multiple Level 1 areas with a Level 2 backbone. If deploying a hierarchy, when and where to create area boundaries become important considerations.

• Addressing ” Both IP and CLNP (NSAP) addresses are needed in the IS-IS network, and assignment of both types of addresses should be well planned.

• Summarization ” This is a key consideration, both from the standpoint of constraining routing instabilities as well as reducing the amount of routing information that is advertised in the network. Selection of the points in the network at which to summarize routing information is usually influenced by the location of area boundaries. Summarization points are not always obvious and related decisions are, therefore, important early in the design process.

Each of these factors needs to be thoroughly explored in the planning stages of the network and re-evaluated during the deployment stages to test viability of any assumptions made earlier. This is important to successfully achieve the ultimate design objectives of the network. Each of these factors is further discussed in detail in this chapter.