Question 103 of 2,152

Route Redistribution →mediumMultiple ChoiceObjective-mapped

Missing EIGRP External Routes in OSPF — Need 'match external'

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.

A
The redistribute eigrp command under OSPF is missing the subnets keyword.
Correct. The missing 'subnets' keyword prevents subnetted routes (including external routes) from being redistributed into OSPF.
B
The redistribute eigrp command under OSPF is missing the match external keyword.
Why wrong: Incorrect. The 'match external' keyword is not a valid option for the 'redistribute eigrp' command under OSPF. The correct command does not have such a keyword.
C
EIGRP has a higher administrative distance than OSPF, causing route suppression.
Why wrong: Incorrect. Administrative distance does not affect redistribution; it is used for route selection when multiple routes exist.
D
The OSPF process on R2 has a distribute-list blocking these routes.
Why wrong: Incorrect. While a distribute-list could block routes, it is less likely than the missing 'subnets' keyword, especially given the symptom of external routes not appearing.

Full breakdown with real-world context →

Quick Answer

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.

Answer choices

Why each option matters

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 subnets keyword.

When redistributing routes into OSPF, the 'subnets' keyword is required to include subnetted routes. Without it, OSPF only redistributes classful network routes, which can cause external EIGRP routes (which are often subnetted) to be missing from the OSPF database. The 'match external' keyword is not a valid option for the 'redistribute eigrp' command under OSPF.

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 this is correct
Correct. The missing 'subnets' keyword prevents subnetted routes (including external routes) from being redistributed into OSPF.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
✗
The redistribute eigrp command under OSPF is missing the match external keyword.
Why it's wrong here
Incorrect. The 'match external' keyword is not a valid option for the 'redistribute eigrp' command under OSPF. The correct command does not have such a keyword.
✗
EIGRP has a higher administrative distance than OSPF, causing route suppression.
Why it's wrong here
Incorrect. Administrative distance does not affect redistribution; it is used for route selection when multiple routes exist.
✗
The OSPF process on R2 has a distribute-list blocking these routes.
Why it's wrong here
Incorrect. While a distribute-list could block routes, it is less likely than the missing 'subnets' keyword, especially given the symptom of external routes not appearing.

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
Incorrect. The 'match external' keyword is not a valid option for the 'redistribute eigrp' command under OSPF. The correct command does not have such a keyword.
Command / output trap
Incorrect. The 'match external' keyword is not a valid option for the 'redistribute eigrp' command under OSPF. The correct command does not have such a keyword.

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.

Visual reference

Quick reference

Routing Protocol Comparison

Protocol	Metric	Max Hops	Algorithm	Type
RIP v2	Hop count	15	Bellman-Ford	Distance vector
OSPF	Cost (bandwidth)	Unlimited	Dijkstra (SPF)	Link state
EIGRP	Composite metric	Unlimited	DUAL	Hybrid
IS-IS	Cost	Unlimited	Dijkstra	Link state
BGP	Policy / attributes	Unlimited	Path vector	Path vector

RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.

What to study next

Got this wrong? Here's your next step.

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.

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FAQ

Questions learners often ask

What does this 300-410 question test?

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 subnets keyword. — When redistributing routes into OSPF, the 'subnets' keyword is required to include subnetted routes. Without it, OSPF only redistributes classful network routes, which can cause external EIGRP routes (which are often subnetted) to be missing from the OSPF database. The 'match external' keyword is not a valid option for the 'redistribute eigrp' command under OSPF.

What should I do if I get this 300-410 question wrong?

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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