See the following figure. All routers on the network run IS-IS and are in an area 49.0001. By referring to the LSDB of R1, the Level-2 DIS is. (the device name, for example, R1.) A. R1 B. R2 C. R3…

Question

Options

• AR1
• BR2
• CR3
• DR4

• A
  75% (46)
• B
  7% (4)
• C
  3% (2)
• D
  15% (9)

Explanation

R1 is the Level-2 DIS because, in IS-IS DIS election on a broadcast segment, the router with the highest interface priority wins; if priorities are equal, the highest SNPA (MAC address) is the tiebreaker. In the figure, R1 holds the highest election metric on that LAN segment, and its LSDB confirms this: you will see a pseudonode LSP with R1's System ID and a non-zero circuit ID (e.g., R1.01-00), which only the DIS generates.

Why the distractors are wrong: R2, R3, and R4 each have a lower interface priority or a lower MAC address than R1, so they lose the election. A non-DIS router does not originate a pseudonode LSP - it only has its own node LSP - so their System IDs will not appear as a pseudonode entry in R1's LSDB.

Memory tip: "The DIS owns the pseudonode - look for the .01 LSP in the LSDB to identify who won the election. Highest priority → highest MAC → winner."

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