Which of the following statements are true about IS-IS addressing? (Select two.)

Question

Options

• ARouters within an area must use unique area addresses.
• BThe area address is used in Level 1 routing.
• CThe system ID identifies the area.
• DThe system ID for Level 1 routers must be unique within an area.
• EThe system ID for Level 2 routers must be unique within the whole domain.

• A
  2% (1)
• B
  7% (4)
• C
  2% (1)
• D
  89% (50)

Explanation

IS-IS uses NETs (Network Entity Titles) structured as Area Address + System ID + SEL (always 00). Option D is correct: Level 1 routers route within an area, so their System IDs must be unique within their area to avoid routing ambiguity among intra-area neighbors. Option E is correct: Level 2 routers participate in inter-area (backbone) routing across the entire IS-IS domain, so their System IDs must be unique across the whole domain to prevent routing loops and forwarding errors. Option A is false - routers within the same area share the same area address; it is the System ID that must be unique. Option B is false - the area address is used to determine area membership and for Level 2 routing decisions, while Level 1 routing uses the System ID portion. Option C is false - the System ID identifies the individual router, not the area; the area address identifies the area.

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