As shown in the figure, all routers run IS-IS. R3 is a Level-1 router, R6 is a Level1-2 router, and other routers are Level-1-2 routers. What are the types of neighbor relationships between routers?

Question

Options

• ALevel-2: R1 and R2; Level-1: R1 and R3, R2 and R5; Unable to establish neighbor relationship: R4 and R6.
• B
• C
• D

• A
  75% (49)
• B
  6% (4)
• C
  3% (2)
• D
  15% (10)

Explanation

Option A correctly reflects IS-IS neighbor formation rules based on router levels and direct connectivity. R1 and R2 are both Level-1-2 routers but sit in different IS-IS areas, so they form only a Level-2 adjacency. R1 and R3 share the same area, and since R3 is Level-1 only, the pair forms a Level-1 adjacency; similarly, R2 and R5 are in the same area as Level-1-2 routers, forming a Level-1 adjacency. R4 and R6 cannot establish any neighbor relationship because they are not directly connected in the topology shown - IS-IS adjacencies only form over direct links, regardless of router level. The distractors are wrong because they either misassign the adjacency level (confusing intra-area Level-1 pairs with Level-2) or incorrectly claim R4 and R6 can form a relationship when no direct link exists.

Memory tip: For IS-IS neighbor rules, remember two filters - first, do the routers share a direct link? If not, no adjacency is possible. Second, do they share an area? If yes, Level-1 is possible; if they also both support Level-2, that adjacency can form too. A Level-1-only router is confined strictly to intra-area (Level-1) relationships.

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