352-001 Question #75: Real Exam Question with Answer & Explanation

The correct answer is B: through R2, because it is an intra-area path. OSPF selects routes based on route type before comparing metrics - intra-area routes (O) are always preferred over inter-area routes (O IA), so R1 uses R2's intra-area path regardless of cost.

Question

Refer to the exhibit. Traffic in this network that is destined for 10.1.3.1 arrives at R1. Which path will the traffic take from here and why?

Exhibit

Options

Athrough R3, because it is the lowest cost path
Bthrough R2, because it is an intra-area path
Cthrough R2, because R3 is in a different autonomous system
Dthrough R3, because R1 will only have a summary (type 3) LSA from R2

Explanation

OSPF selects routes based on route type before comparing metrics - intra-area routes (O) are always preferred over inter-area routes (O IA), so R1 uses R2's intra-area path regardless of cost.

Common mistakes.

A. OSPF does not use cost as the primary selection criterion when paths of different route types are available - a lower-cost inter-area path will always lose to a higher-cost intra-area path.
C. The exhibit does not show R3 in a different autonomous system - the relevant distinction is intra-area vs. inter-area, not intra-AS vs. external, and incorrectly framing R3 as an ASBR misidentifies the topology.
D. This is self-contradicting - if R2 were only advertising a Type 3 LSA (inter-area summary), R2's path would be inter-area, not intra-area, which contradicts the correct answer establishing R2 as the preferred intra-area path.

Concept tested. OSPF route type preference - intra-area over inter-area

Reference. https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/7039-1.html

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