The IS-IS hierarchy should place access or internal area routers at Level 1, boundary routers at Level 1/2, and backbone routers at Level 2. In this exhibit, users in areas 25 and 55 must send and receive traffic through both backbone areas, while link flaps in areas 35 and 45 must not affect other areas. Cisco IS-IS design uses Level 1 areas to contain local topology changes and Level 2 routing to interconnect areas through the backbone. Level 1/2 routers sit at the area boundary and leak or advertise reachability between the local Level
1 area and the Level 2 backbone. Option C matches that hierarchy by placing A-series routers as Level 1, B- series routers as Level 1/2, and C-series routers as Level 2. Making too many routers Level 1/2 would expand the impact of topology changes and reduce scaling benefits. Making backbone routers Level 1 would break interarea design. Reference topics: IS-IS hierarchy, Level 1, Level 2, Level 1/2 routers, failure-domain containment, enterprise scaling.