Which of the following is the Link ID of the third type LSA?

Question

Options

• ARouter ID of the described ABR
• BThe port IP address of the ABR on the network segment where it is located
• CThe destination network segment described
• DThe router ID of the router where the LSA originated

• A
  7% (3)
• B
  14% (6)
• C
  77% (34)
• D
  2% (1)

Explanation

In OSPF, a Type 3 LSA (Summary LSA) is generated by an ABR (Area Border Router) to advertise inter-area routes. Its Link ID field identifies the destination network being summarized and flooded into other areas - that network prefix is what other routers need to build their routing tables, making C correct.

Why the distractors are wrong:

• A (Router ID of the ABR): The ABR's router ID appears in the Advertising Router field of the LSA header, not the Link ID. Confusing these two fields is a common trap.
• B (Port IP of the ABR on the segment): An interface IP of the DR is used as the Link ID in Type 2 (Network) LSAs, not Type 3. Each LSA type has its own Link ID semantics.
• D (Router ID of the originating router): This describes the Link ID for Type 1 (Router) LSAs, where a router advertises itself by its own Router ID.

Memory tip: Map the LSA type to what it's describing - Type 1 describes a router (Link ID = Router ID), Type 2 describes a network segment via its DR (Link ID = DR interface IP), and Type 3 describes a destination network being summarized across areas (Link ID = network prefix). Each Link ID answers: "What is this LSA about?"

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