The networking diagram of a certain IS-IS is as shown in the figure. According to the content analysis in the figure, which one of the following descriptions is wrong?

Question

Options

• AThe LSDB of Level-2 of the R6 device only contains the routing information of this area.
• BThe LSDB of Level-1 of the R1 device only contains the routing information of this area.
• CR3 and R1 form a Level-1 adjacency relationship, and R2 and R4 form a Level-2 adjacency relationship.
• DR1 can access R4 through R2 or R3

• A
  73% (40)
• B
  16% (9)
• C
  7% (4)
• D
  4% (2)

Explanation

Option A is wrong because in IS-IS, the Level-2 LSDB is the backbone database and contains routing information for the entire IS-IS domain - all areas - not just the local area. Level-2 routing exists precisely to enable inter-area communication, so R6's L2 LSDB would include topology information from every area in the domain.

Option B is correct (not wrong) because a Level-1 LSDB genuinely contains only the routing information of its own area; L1 routers are area-local by design, so this statement accurately describes IS-IS behavior.

Option C is correct because in a typical IS-IS topology where R1 and R3 share the same area they form an L1 adjacency, while R2 and R4 operating at the backbone level form an L2 adjacency - both relationships are consistent with standard IS-IS operation.

Option D is correct because R1 has two valid paths to reach R4 in the described topology, either through R2 directly or via R3, which is consistent with IS-IS path selection.

Memory tip: Use "L1 = Local, L2 = Large domain." The Level-1 database is confined to the local area, while the Level-2 database spans the whole IS-IS domain. Any answer claiming the L2 database is area-limited is always wrong.

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