IS-IS has always been my favourite Interior Gateway Protocol. Its elegant simplicity, its well-structured data formats, its ?exibility and easy extensibility are all appealing IS-IS epitomizes link-state routing. Whether for this reason or others, IS-IS is the IGP of choice in some of the world s largest networks. Thus, if one is at all interested in routing, it is well worth the time and effort to learn IS-IS. However, it is hazardous to call any routing protocol simple . Every design decision, be it in architecture, implementation or deployment, has consequences, some unanti- pated, some...