IS-IS was designed to carry node addresses (NSAPs) between level-1 routers (called Intermediate Systems) within an area and area prefixes between level-2 routers, resulting in a perfect separation of concerns and forwarding information summarization. When IETF tried to use the same routing protocol for a networking stack with a completely different addressing mentality, something had to give.

By default, IS-IS in the IP world (like OSPF) does not do prefix summarization, and the summarization ranges must be configured by the network operator. Want to try out how that works? The Summarizing Level-1 Routes into Level-2 Backbone lab exercise is waiting for you.

If you already set up your own netlab environment, you probably know what to do (or you can get the details here). Alternatively, you can click here to start the lab in your browser using GitHub Codespaces. After starting the lab environment, change the directory to advanced/3-summarization and execute netlab up.