If you design multi level IS-IS network and if you have more than one exit (L1-L2 routers) from the Level 1 domain, you will likely create a suboptimal routing. Multi-level IS-IS design is for large-scale network. What’s more, most of the real life networks use only flat Level 2 IS-IS as their interior gateway protocol (IGP).
In the figure shown above, Router A is in the Level 1 IS-IS domain. While Router B is in the Level 1 IS-IS domain, it is in different area. Router A has two exit point/default gateway (L1-L2 Routers) to reach Router B; however, unlike Router B, Router A operates in different domain.
In IS-IS Level 1-Level 2, routers don’t send anything except default route (with ATT bit in the Level 1 LSP) towards the internal Level 1 Routers. Thus, Router B only trust it’s Level 1-Level 2 router information.
In addition, Router A doesn’t know the entire IS-IS topology would be in the flat Level 2 IS-IS topology. Furthermore, both L1-L2 routers advertise the same subnet – towards Router A – for the Router B network. In fact, only the metric of Router B is different. Left L1-L2 gateway sends the route of Router B with metric 5; right L1-L2 sends the route of Router B with metric 10.
Because of that, Router A chooses the Left L1-L2 router as its exit point/default gateway to reach Router B. Obviously, this creates sub-optimal routing since the left L1-L2 router sends the packet to top routers so that more hops packet can travel – an effect that we don’t want in our design.
Sub-optimal routing is appealing if you know the requirements of the application. Some applications can tolerate suboptimal routing since their timeout, delay, and jitter expectation may not be sensitive. In sum, putting the low-end devices into an L1 domain provides fault isolation, which in turn provides scalability.