How does a Level 1 IS-IS router reach destinations outside its own area?

A Level 1 IS-IS router operates within its own area and is primarily responsible for routing within that area. To reach destinations outside its own area, it must rely on Level 1/2 routers, which have knowledge of both Level 1 and Level 2 routing. Level 1/2 routers act as boundary routers that connect different areas and facilitate communication between them.

The correct approach for a Level 1 IS-IS router to reach external destinations is to send traffic to the nearest Level 1/2 router. This router will handle the next steps in the routing process, allowing the data packets to traverse to other areas where the desired destination resides. By utilizing the Level 1/2 router, the Level 1 router can effectively expand its reachability beyond its local area.

Other options, such as routing through the closest Level 1 router or designated intermediate systems, do not account for the necessary capabilities that Level 1/2 routers possess to manage inter-area routing, making them less effective for this purpose.