Level 1 routers store information about all the subnets within an area, and choose intranetwork paths over internetwork paths. Using the area ID portion of the NET address, Level 1 routers determine which neighboring routers are Level 1 routers within the same area.
If the destination address is not within the area, Level 1 routers forward the packet to the nearest router configured as both a Level 1 and Level 2 router within the area. The Level 1 and Level 2 router forwards the packet, using the Level 2 topology, to the proper area. The destination router, which is configured as a Level 1 and Level 2 router, then determines the best path through the destination area.