Question 11 of 2,152

OSPF Troubleshooting (v2/v3) →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is a distribute-list applied to the OSPF process that filters the route 10.0.0.0/8. When a Type 5 LSA for an external route appears in the OSPF database but is missing from the routing table, the router has received and processed the LSA but is blocking the route installation at the RIB level. This occurs because distribute-list filtering operates after the SPF calculation, preventing the route from being installed even though the LSA is fully recognized. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of the distinction between the OSPF database and the routing table, a common trap where candidates assume a missing route means a missing LSA. Remember the memory tip: "Database present, route absent—check the distribute-list for the present absent."

300-410 OSPF Troubleshooting (v2/v3) Practice Question

This 300-410 practice question tests your understanding of ospf troubleshooting (v2/v3). 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 an OSPFv2 issue where a router R1 is not receiving a specific route for 10.0.0.0/8 from a neighbor R2. The adjacency is FULL, and 'show ip ospf database' on R1 shows the LSA for 10.0.0.0/8 as a type 5 LSA. However, the route is not in the routing table. 'show ip route 10.0.0.0' shows no route. 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.

Question 1hardmultiple choice

Review the full OSPF breakdown →

A
The router R1 has a distribute-list applied to the OSPF process that filters the route 10.0.0.0/8.
A distribute-list can prevent a route from being installed in the routing table even if the LSA is in the database.
B
The route is a type 5 LSA, but the router is in a stub area.
Why wrong: Stub areas block type 5 LSAs, so the LSA would not be in the database.
C
The router has a higher administrative distance for OSPF routes, causing it to prefer another routing protocol.
Why wrong: If there were another route, it would appear in the routing table; the stem says no route is present.
D
The route has a metric of 16777214, which is considered infinite.
Why wrong: A metric that high would still be installed, but the router would not use it; however, the route would appear in the routing table as a candidate.

Full breakdown with real-world context →

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 router R1 has a distribute-list applied to the OSPF process that filters the route 10.0.0.0/8.

If the LSA is in the database, the route should be installed unless there is a filtering mechanism or a routing table issue. One common cause is that the router has a route map that filters the route from being installed. Another possibility is that the router has a static route that is being preferred, but the stem says no route is present. The most likely cause is that the router has a distribute-list that filters the route.

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 router R1 has a distribute-list applied to the OSPF process that filters the route 10.0.0.0/8.
Why this is correct
A distribute-list can prevent a route from being installed in the routing table even if the LSA is in the database.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
✗
The route is a type 5 LSA, but the router is in a stub area.
Why it's wrong here
Stub areas block type 5 LSAs, so the LSA would not be in the database.
✗
The router has a higher administrative distance for OSPF routes, causing it to prefer another routing protocol.
Why it's wrong here
If there were another route, it would appear in the routing table; the stem says no route is present.
✗
The route has a metric of 16777214, which is considered infinite.
Why it's wrong here
A metric that high would still be installed, but the router would not use it; however, the route would appear in the routing table as a candidate.

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.

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?

OSPF Troubleshooting (v2/v3) — This question tests OSPF Troubleshooting (v2/v3) — OSPF neighbours must agree on key parameters..

What is the correct answer to this question?

The correct answer is: The router R1 has a distribute-list applied to the OSPF process that filters the route 10.0.0.0/8. — If the LSA is in the database, the route should be installed unless there is a filtering mechanism or a routing table issue. One common cause is that the router has a route map that filters the route from being installed. Another possibility is that the router has a static route that is being preferred, but the stem says no route is present. The most likely cause is that the router has a distribute-list that filters the route.

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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Same concept, more angles

1 more ways this is tested on 300-410

These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.

Variation 1. A network engineer is troubleshooting an OSPFv3 issue where a router R1 is not learning routes from a neighbor R2. The adjacency is FULL, but 'show ipv6 route ospf' on R1 shows only a default route. R2 is an ASBR redistributing connected routes into OSPFv3. 'show ipv6 ospf database external' on R1 shows the external routes, but they are not installed in the routing table. What is the most likely cause?

hard

✓ A.The router R1 has a distribute-list in the OSPFv3 process that filters out the external routes.
B.The external routes have a metric of 16777214, which is considered infinite.
C.The router R1 does not have IPv6 unicast routing enabled.
D.The external routes are type 5 LSAs but the router is in a totally stubby area.

Why A: In OSPFv3, external routes are not installed if the router has a route to the forwarding address that is not reachable. However, the more common cause is that the OSPFv3 process on R1 has 'no ipv6 unicast-routing' or the router is missing a default route to reach the external destinations. Another possibility is that the external routes have a metric that is too high or the route is suppressed by a filter. The most likely cause is that the router has a route map or distribute list that is filtering the external routes from being installed.

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