Different types of LSAs have different functions. Which of the following LSAs can only be spread within a single area? (Multiple choice)

Question

Options

• ANetwork Summary LSA
• BRouter LSA
• CNetwork LSA
• DAS-External-LSA

• A
  7% (1)
• B
  73% (11)
• D
  20% (3)

Explanation

Router LSAs (Type 1) and Network LSAs (Type 2) are both confined to the area in which they originate - they are never forwarded across area boundaries by Area Border Routers (ABRs). Type 1 describes a router's links within an area, and Type 2 describes a multi-access network's attached routers; both pieces of information are only meaningful inside that specific area's topology.

Why the distractors are wrong:

• A (Network Summary LSA, Type 3): Generated by ABRs specifically to cross area boundaries, advertising inter-area routes to other areas - its entire purpose is inter-area flooding.
• D (AS-External-LSA, Type 5): Generated by ASBRs to advertise external routes and flooded throughout the entire OSPF domain (all non-stub areas), making it the broadest scope of all.

Memory tip: Think "1 and 2 stay true" - Type 1 (Router) and Type 2 (Network) LSAs are the two that stay within their area. Everything above Type 2 crosses some boundary: Type 3/4 cross area borders, Type 5 crosses the AS boundary.

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