The answer is that OSPF prefers inter-area routes over external routes, which is why R3 chooses the route via R1 even though R2’s type-5 LSA has a lower metric. In OSPF, the route selection order is intra-area (O) first, then inter-area (O IA), and finally external (O E1/E2 or N1/N2). When R1, acting as an ABR, redistributes the iBGP-learned prefix into OSPF, it generates a type-3 inter-area LSA, while R2’s redistribution produces a type-5 external LSA. Since inter-area routes are inherently preferred over external routes regardless of metric, R3 installs the route via R1. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of OSPF route preference hierarchy and the subtle distinction between redistribution behavior on an ABR versus a non-ABR. A common trap is assuming metric alone decides the winner, but OSPF’s LSA type precedence overrides cost. Memory tip: “I before E, except after C” — Inter-area beats External, and cost only matters within the same LSA type.
300-410 Administrative Distance Practice Question
This 300-410 practice question tests your understanding of administrative distance. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Router R1 and R2 are iBGP peers. R1 learns a route 10.10.10.0/24 from an eBGP peer with local preference 200. R2 learns the same route from another eBGP peer with local preference 150. Both routers redistribute the route into OSPF with default settings. R3, an OSPF router, receives two type-5 LSAs for 10.10.10.0/24: one from R1 with metric 20, one from R2 with metric 10. R3's 'show ip route 10.10.10.0' shows the route via R1. What is the root cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The route from R1 is an inter-area route (type-3) while the route from R2 is an external route (type-5); OSPF prefers inter-area over external.
OSPF external routes are compared by metric (cost) first for type-5 routes. The route from R2 has metric 10, which is lower than R1's metric 20, so R3 should prefer R2. However, if the administrative distance is changed, it could affect preference. The correct answer is that the route from R1 might have a lower administrative distance if R1 is an ABR and the route is an inter-area route (AD 110) while R2's route is external (AD 110) but with a different metric. But both are external. The trick is that R1's route might be an NSSA external route (type N2) with AD 110, but metric comparison still applies. The correct answer is that the route from R1 is actually an inter-area route (type-3) because R1 is an ABR, and inter-area routes have AD 110 but are preferred over external routes of the same AD? No, OSPF prefers intra-area > inter-area > external. So if R1's route is inter-area (AD 110) and R2's is external (AD 110), inter-area is preferred. This is the most likely.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
The route from R1 is an inter-area route (type-3) while the route from R2 is an external route (type-5); OSPF prefers inter-area over external.
Why this is correct
OSPF route selection prefers intra-area, then inter-area, then external type-1, then external type-2. Inter-area is preferred over external.
R3's OSPF process has 'distance ospf external 200' configured, making R2's route less preferred.
Why it's wrong here
No such configuration mentioned.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
Administrative Distance — This question tests Administrative Distance — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The route from R1 is an inter-area route (type-3) while the route from R2 is an external route (type-5); OSPF prefers inter-area over external. — OSPF external routes are compared by metric (cost) first for type-5 routes. The route from R2 has metric 10, which is lower than R1's metric 20, so R3 should prefer R2. However, if the administrative distance is changed, it could affect preference. The correct answer is that the route from R1 might have a lower administrative distance if R1 is an ABR and the route is an inter-area route (AD 110) while R2's route is external (AD 110) but with a different metric. But both are external. The trick is that R1's route might be an NSSA external route (type N2) with AD 110, but metric comparison still applies. The correct answer is that the route from R1 is actually an inter-area route (type-3) because R1 is an ABR, and inter-area routes have AD 110 but are preferred over external routes of the same AD? No, OSPF prefers intra-area > inter-area > external. So if R1's route is inter-area (AD 110) and R2's is external (AD 110), inter-area is preferred. This is the most likely.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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