One of the major differences between OSPF and IS-IS is their handling of inter-area routes. Non-backbone OSPF intra-area routes are copied into the backbone area and later (after the backbone SPF run) copied into other areas. IS-IS does not copy level-2 routes into level-1 areas; level-1 areas (by default) behave like totally stubby OSPF areas with the level-1 routers using the Attached (ATT) bit of level-1-2 routers in the same area to generate the default route.

As always, there’s a nerd knob to change that behavior, and you can explore it (and its consequences) in the Distributing Level-2 IS-IS Routes into Level-1 Areas lab exercise created by Dan Partelly.

Click here to start the lab in your browser using GitHub Codespaces (or set up your own lab infrastructure). After starting the lab environment, change the directory to advanced/2-route-leak and execute netlab up.