All routers in a network run IS-IS and are all in Area 49.0001. The LSDB of R1 is as shown in the figure. Based on this, match the Level of the four routers with their corresponding router IDs. The…

Question

Options

• ALevel-2 maps to R1; Level-1 maps to R2; Level-1-2 maps to R3; Level-1-2 maps to R4
• BNot provided in the source.
• CNot provided in the source.
• DNot provided in the source.

• A
  79% (19)
• B
  13% (3)
• C
  4% (1)
• D
  4% (1)

Explanation

In IS-IS, a router's operating level is revealed by where its System ID appears in a neighbor's LSDB: a System ID found only in the Level-1 database belongs to a Level-1 router, one found only in the Level-2 database belongs to a Level-2 router, and one found in both databases belongs to a Level-1-2 router. Applying this to R1's LSDB: R4 (0001.0000.4004.00) appears only in the Level-1 entries, making it Level-1; R3 (0001.0000.3003.00) appears only in the Level-2 entries, making it Level-2; R2 (0001.0000.2002.00) appears in both the Level-1 and Level-2 entries, making it Level-1-2; and R1 (0001.0100.1001.00, marked with * as self-originated) also appears in both databases, making R1 itself Level-1-2 as well.

Note of caution: The text of option A as presented here states "Level-2 maps to R1; Level-1 maps to R2; Level-1-2 maps to R3; Level-1-2 maps to R4," which does not match the LSDB data as described above. The IS-IS analysis of the described data yields the opposite: R1 = Level-1-2, R2 = Level-1-2, R3 = Level-2, R4 = Level-1. This suggests the option text was likely mis-transcribed and option A in the original exam probably states that mapping. The methodology above is what the exam is testing.

Memory tip: Think of it as "which pool does the router swim in?" - Level-1 only swims in the L1 pool, Level-2 only swims in the L2 pool, and Level-1-2 swims in both. If you see a System ID in both pools of a neighbor's LSDB, that router is a Level-1-2 border router connecting the area to the backbone.

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