The answer is a missing `match external` keyword on the `redistribute eigrp` command under OSPF. This is correct because, by default, when redistributing from EIGRP into OSPF, only EIGRP internal routes are advertised; EIGRP external routes—those originally redistributed into EIGRP from another protocol—are excluded unless the `match external` keyword is explicitly added. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of the granular control EIGRP offers over route redistribution, often appearing as a troubleshooting question where `show ip ospf database external` reveals missing prefixes. A common trap is assuming all EIGRP routes are treated equally, but the default behavior only matches internal routes. To remember: think of EIGRP as having two “buckets”—internal and external—and OSPF’s redistribution only dips into the internal bucket unless you tell it to “match external” as well.
300-410 Route Redistribution Practice Question
This 300-410 practice question tests your understanding of route redistribution. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
A network engineer is troubleshooting a route redistribution issue between EIGRP and OSPF. Routers R1 (EIGRP) and R2 (OSPF) are redistributing routes. The engineer notices that some EIGRP external routes (redistributed into EIGRP from another protocol) are not appearing in the OSPF database on R2. The show ip ospf database external command on R2 does not list these prefixes. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The redistribute eigrp command under OSPF is missing the match external keyword.
When redistributing from EIGRP into OSPF, by default only EIGRP internal routes are redistributed; EIGRP external routes are not included unless the match external keyword is specified.
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 redistribute eigrp command under OSPF is missing the subnets keyword.
Why it's wrong here
The subnets keyword is needed for classless routes, but the issue here is about external routes, not subnetting.
✓
The redistribute eigrp command under OSPF is missing the match external keyword.
Why this is correct
Correct: Without match external, EIGRP external routes are not redistributed into OSPF.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
EIGRP has a higher administrative distance than OSPF, causing route suppression.
Why it's wrong here
Administrative distance affects route selection, not redistribution of specific route types.
✗
The OSPF process on R2 has a distribute-list blocking these routes.
Why it's wrong here
While possible, the default behavior of not redistributing external routes is a more common and likely cause.
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.
Trap categories for this question
Keyword trap
The subnets keyword is needed for classless routes, but the issue here is about external routes, not subnetting.
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.
Route Redistribution — This question tests Route Redistribution — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The redistribute eigrp command under OSPF is missing the match external keyword. — When redistributing from EIGRP into OSPF, by default only EIGRP internal routes are redistributed; EIGRP external routes are not included unless the match external keyword is specified.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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