Summary

This chapter presents various case studies to elaborate on various network design issues when IS-IS is deployed as an IGP. The chapter starts by reviewing two migration scenarios that involve making changes in existing IS-IS configurations. The first is concerned with migration of IS-IS areas from one area ID to another, and the second deals with migration from narrow to wide metrics.

Next, IS-IS configurations are reviewed for point-to-point links and for NBMA transport deployed in ISP environments. A great amount of time is dedicated to issues relating to deploying IS-IS in NBMA environments with highly meshed PVCs. Common configuration problems and solutions are reviewed. The reviews indicate the best model for NBMA links in IS-IS networks is the point-to-point model in which point-to-point subinterface is used to model the virtual circuits as point-to-point links. IS-IS mesh groups are also suggested as a solution to address scaling issues posed by redundant flooding in highly meshed environments using the point-to-point model.

A substantial part of the chapter focuses on migrating from other IGPs to IS-IS. Two approaches are recommended: The Redistribution and Background methods . An elaborate example based on migration from EIGRP is discussed. The whole process is also summarized into four phases. Migration from OSPF is briefly discussed, and the Background method emerged here also as the recommended approach. Specific considerations for OSPF are highlighted.

The four-phase migration approach based on the Background method was also identified to be applicable for migrating from classful protocols, such as RIP and IGRP.

The final sections of the chapter were focused on two important issues, namely coexistence of IS-IS with other IP IGPs and using IS-IS in dual environments. Two case studies that were focused on configuration issues when mixing ISO CNLS and IP routing in Dual domains are presented to reinforce earlier discussions on RFC1195 requirements.