Which of the following descriptions of the OSPF asbr-summary command is incorrect?

Question

Options

• AThe Asbr-summary command can only take effect when deployed on the ASBRs that need to be aggregated.
• BThe routes summarized using the asbr-summary command will be transmitted within the area as type 5 LSA.
• CRoutes summarized using the asbr-summary command can be flooded to other areas.
• DThe asbr-summary command can summarize LSAs aggregated by asbr-summary.

• A
  6% (3)
• B
  16% (8)
• C
  4% (2)
• D
  75% (38)

Explanation

Option D is incorrect because the asbr-summary command cannot be used to further aggregate routes that have already been summarized by another asbr-summary statement - it operates only on the original, unaggregated external routes, not on previously summarized prefixes. Option A is a correct description: the command must be configured directly on the ASBR that is redistributing external routes into OSPF; without placement on that specific router, the summarization has no effect. Option B is also correct: the ASBR advertises the aggregated prefix as a Type 5 LSA (AS External LSA), which is the standard LSA type for external routes in OSPF. Option C is likewise correct: Type 5 LSAs are flooded throughout the entire OSPF domain, crossing area boundaries into all non-stub areas, so the summarized route is visible domain-wide.

Memory tip: Think of asbr-summary as a "one-pass condenser" - it compresses raw external routes into a single Type 5 LSA on the ASBR, but it cannot compress a summary that is already compressed. If you could chain asbr-summary commands recursively, that would be the incorrect behavior described in D.

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