The four routers run IS-IS and have established adjacency. The area numbers and router levels are shown in the figure. Which of the following is correct?

Question

Options

• AThe LSP of R4 does not exist in the LSDB of R3 router
• BThe LSP of R4 does not exist in the LSDB of the R2 router
• CThe LSP of R4 does not exist in the LSDB of the R1 router
• DThe LSP of R1 exists in the LSDB of R4 router

• A
  4% (1)
• B
  4% (1)
• C
  11% (3)
• D
  82% (23)

Explanation

D is correct because in IS-IS, Level 2 (L2) LSPs are flooded across the entire L2 domain. Since R4 is an L2 or L1/L2 router, it maintains an L2 LSDB containing LSPs from all L2-capable routers-including R1-regardless of which area R1 belongs to. The L2 flooding domain crosses area boundaries, so R1's LSP always reaches R4.

Distractors A, B, and C are all false because they each claim R4's LSP is absent from another router's LSDB. Since all four routers are L2 or L1/L2 capable, R4's L2 LSP is flooded to-and stored by-R3, R2, and R1. No router in the L2 domain is excluded from receiving any other L2 router's LSP.

Memory tip: Think of L2 IS-IS as a "global broadcast club." Every L2/L1/L2 router shares its LSP (membership card) with every other L2-capable router across all areas-pure L1 routers are the only ones excluded. If a router speaks L2, it knows about all other L2 speakers.

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