In IS-IS, into how many levels can the routing domain be divided?

• A. 1
• B. 2
• C. 3
• D. 4

Answer: (B)

IS-IS uses a two-level hierarchy to separate intra-area and inter-area routing. Level-1 handles routing inside a single area, so those routers know the paths to destinations that are within the same area but not how to reach other areas. Level-2 forms the backbone that connects different areas and carries routing information between them, allowing reachability across the entire IS-IS domain. Routers that need both capabilities can run both levels (Level-1 and Level-2), becoming L1/L2 routers that participate in both planes. There isn’t a third level in the standard IS-IS design, so the routing domain is divided into two levels.