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The Complete IS-IS Routing Protocol- P51: IS-IS has always been my favourite Interior Gateway Protocol. Its elegant simplicity, its well-structured data formats, its flexibility 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. | 502 16. Network Design except for the bare minimum needed to bring up the iBGP transport mesh. Once the iBGP transport mesh has been brought up BGP carries all IP information including linklocal IP sub-net prefixes. That design choice is no real choice - it is paramount - all the other options depend on what you expect from the network and what is doable with the prevailing hardware software combination. Ultimately one has to listen to what endcustomers are expecting and what kind of services have to be modelled on top of that backbone. For good network designs there are two important insights. First you always have to make a compromise. Networks are complex and changes of any kind have multidimensional impact. If you optimize on one direction then you deliberately have to jeopardize the other. So pick carefully which of the three areas you want to optimize on scalability stability or convergence. While it would be a highly desirable goal to maximize all three in practice you can only optimize on two. The second insight is an old rule of systems design - keep it simple. Before exploring complex matters of route leaking and multi-level designs first answer the question as to whether you really need all that complexity. Would the network work even if you go with a single level design and what would be the overall scalability impact Practical experience says that modern IS-IS implementations are more scalable than people think and keep some sanity sometimes network designers try to seek solutions for non-existent problems. 17 Future of IS-IS Writing a book about IS-IS is a never-ending task. During the writing of this book a lot that was originally planned for this future chapter got implemented in routing software and is now available and deployed in the Internet. It became clear to the authors that whatever we put into this chapter would just be a snapshot of the thoughts and published Internet drafts at one particular time. You will find a lot of proposals here .
