Which of the following descriptions about LSA and LSDB are wrong? (Multiple choice)

Question

Options

• AThe LS sequence number field is used to detect old and duplicate LSAS
• BThe combination of LS type and Link StateID jointly identifies an LSA
• CLSA is not resident until in LSDB
• DThe LSage field in the LSA header indicates the remaining lifetime of the LSA

• A
  3% (1)
• B
  86% (25)
• C
  10% (3)

Explanation

Options B and D contain incorrect descriptions of LSA/LSDB behavior in OSPF.

Why B is wrong: An LSA is uniquely identified by three fields - LS Type, Link State ID, and Advertising Router ID. Omitting the Advertising Router is incorrect because multiple routers can generate LSAs of the same type with the same Link State ID (e.g., multiple AS external LSAs), so all three fields together form the unique key.

Why D is wrong: The LSAge field tracks elapsed age, not remaining lifetime. An LSA starts at age 0 when originated and counts upward to MaxAge (3600 seconds), at which point it expires and is flushed. Describing it as "remaining lifetime" reverses the direction entirely.

Why A and C are correct (not wrong): The LS Sequence Number genuinely does detect old/duplicate LSAs by letting routers compare which instance is newer - making A a true statement. LSAs genuinely are stored and reside in the LSDB, making C also true.

Memory tip: For LSAge, think of it like a person's age - it starts at 0 and counts up, not down. For LSA identification, remember the acronym "T-I-R" (Type, ID, Router) - you need all three to pinpoint an LSA, not just two.

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