Which two statements are correct about IS-IS? (Choose two.)

Question

Options

• AA level 1 only router can never form an adjacency with a level 2 only router.
• BFor level 2 adjacencies, the area IDs can be different.
• CFor level 2 adjacencies, the area IDs must be the same.
• DA level 1 only router can form an adjacency with a level 2 only router.

• A
  76% (25)
• C
  9% (3)
• D
  15% (5)

Explanation

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. IS-IS (Intermediate System to Intermediate System) operates at two levels: Level 1 and Level 2. Level 1 routers are only aware of their own area's topology, while Level 2 routers have knowledge of the topology across areas. A Level 1 router cannot form an adjacency with a Level 2 router unless the Level 2 router is also operating as a Level 1 router (Level 1-2 router). Level 2 routers can form adjacencies regardless of their area IDs because Level 2 operates at the domain level and is used to interconnect different IS-IS areas.

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