The Intermediate System-to-Intermediate System (IS-IS) protocol was originally designed to provide routing information for the Open Systems Interconnect (OSI) protocols. IS-IS is a link-state protocol in which Intermediate Systems (ISs), or routers, flood routing information to each other within hierarchical levels.
So why would you want to consider IS-IS for routing in a large-scale IP network? In fact, I'm certain some people out there right now are thinking, "IS-IS—are you crazy? It's so hard to configure."
On the contrary, IS-IS is used in very large-scale IP networks, primarily because of its flexible timers, fast convergence, and capability to handle instability in the IP routing domain very well. It's to IS-IS's advantage, in many cases, that it wasn't originally designed for routing IP, but rather, that it was adapted for IP routing by the Internet Engineering Task Force (IETF). The main advantage is that changes in IP routing information don't affect the core of its functionality, which is to provide Connectionless Network Service (CLNS) routing information.
This chapter works through implementing IS-IS on the network built in Chapter 4, "Applying the Principles of Network Design," so that you can get a feel for the issues involved. There are plenty of case studies in this chapter that cover different aspects of IS-IS's operation, various design options and issues, and some troubleshooting tips.
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