A Level 1 router can become adjacent with the Level 1 and Level 1-2 (L1/L2) router. A Level 2 router can become adjacent with Level 2 or Level 1-2 (L1/L2) router. There is no adjacency between L1 only and L2 only router. HOWEVER: If two routers are in different areas, they can only form a Level 2 adjacency. As such, two routers in different areas can NOT form a Level 1 adjacency. If you want two routers to form a Level 1 adjacency, they have to be in the same area.

AC A. For IS-IS Level 2 adjacencies, the routers must be in the same area. Therefore, the area IDs must be the same for Level 2 adjacencies to form. C. A Level 1 only router can indeed form an adjacency with a Level 2 only router. This is because IS-IS supports inter-area routing, allowing routers in different areas (Level 1 and Level 2) to form adjacencies and exchange routing information.

Correct answer is BD: 1) Level 1 Routers never form an adjacency with Level 2 router and the reverse is also true . 2. For Level 1 adjacencies, Area IDs must be same. 3. For Level 2 adjacencies, Area IDs can be diffrent.