Which of the following descriptions of IS-IS area division are correct? (Multiple choice)

Question

Options

• AIn IS-IS, Level-1-2 routers connect the backbone area and non-backbone area.
• BIn IS-IS, different interfaces of a single router can belong to different areas.
• CAll IS-IS non-backbone areas and all IS-IS backbone area is defined as a backbone area.
• DIn IS-IS, each router can only belong to one area.

• A
  76% (28)
• B
  8% (3)
• C
  16% (6)

Explanation

A is correct because Level-1-2 routers serve as the critical link between Level-1 (non-backbone) areas and the Level-2 backbone in IS-IS - they run both L1 and L2 processes simultaneously, learning intra-area routes via L1 and injecting a default route toward the backbone via L2. D is correct because in IS-IS, a router's area membership is defined by its NET (Network Entity Title) address, which is a single area identifier applied to the entire router - unlike OSPF, IS-IS does not allow different interfaces to reside in different areas.

B is wrong because IS-IS area membership is router-wide, not interface-specific; all interfaces on a given router share the same area address from its NET. C is wrong because it inverts the IS-IS hierarchy - non-backbone (Level-1) areas are explicitly not backbone areas; the backbone is exclusively the Level-2 domain formed by L2 and L1-2 routers.

Memory tip: Contrast IS-IS with OSPF - in OSPF, an ABR literally straddles areas with different interfaces in each area; in IS-IS, the L1-2 router is fully inside one area but speaks two languages (L1 locally, L2 globally). One router = one area, always.

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