On an IS-IS network, Level-1 routers can access the IS-IS backbone area only through Level-1-2...

IS-IS uses a two-level hierarchy:

Level-1 routers operate within a single area and cannot exchange routing information with routers in other areas directly.

To reach external areas (the IS-IS backbone or other areas), Level-1 routers rely on Level-1-2 routers , which perform inter-area route redistribution.

Therefore, a Level-1 router must use a Level-1-2 router as a gateway to reach destinations outside its own area.