Question 1,993 of 2,152

VRF-Lite →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is a unidirectional link issue, where EIGRP packets are successfully sent but not received, causing the query process to time out. This occurs because EIGRP relies on the Reliable Transport Protocol (RTP) to ensure delivery of queries and replies; when one direction of the Ethernet link fails silently—perhaps due to a faulty transceiver or cable—the router sending queries never receives the required acknowledgment or reply, leading to a stuck-in-active (SIA) condition even though the interface shows no errors. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how EIGRP’s query process interacts with VRF-Lite, and it’s a classic trap where candidates mistakenly blame bandwidth or duplex mismatches. Remember the memory tip: “SIA means silence in one direction—check the path, not the link state.”

300-410 VRF-Lite Practice Question

This 300-410 practice question tests your understanding of vrf-lite. 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.

In a VRF-Lite environment, EIGRP is configured between two routers. The engineer notices that the EIGRP neighbor relationship is flapping intermittently. Debug output shows 'dually' messages and the route is occasionally marked as 'stuck-in-active' (SIA). The link is Ethernet with no errors. Which is the most likely explanation?

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

Study the full EIGRP explanation →

A
The EIGRP K-values are mismatched between the two routers, causing the neighbor to reset.
Why wrong: K-value mismatch prevents the neighbor from forming at all, not flapping with SIA.
B
A unidirectional link issue is present, where EIGRP packets are successfully sent but not received, causing the query process to time out.
Unidirectional link causes queries to be sent but replies not received, leading to SIA and neighbor flapping.
C
The EIGRP stub routing feature is enabled on one router, preventing query propagation and causing the active process to hang.
Why wrong: Stub routing prevents queries from being sent to the stub router, but it does not cause SIA; it actually reduces the chance of SIA.
D
The 'eigrp log-neighbor-changes' command is causing excessive logging, which delays EIGRP processing.
Why wrong: Logging does not cause SIA or flapping; it only records events.

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

✓

A unidirectional link issue is present, where EIGRP packets are successfully sent but not received, causing the query process to time out.

EIGRP uses the Reliable Transport Protocol (RTP) for updates, queries, and replies. If there is a unidirectional link issue (e.g., one direction has high latency or packet loss), the query process may not receive replies in time, causing the route to become SIA. This is a classic edge case where the link appears operational but is unidirectional for EIGRP packets.

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 EIGRP K-values are mismatched between the two routers, causing the neighbor to reset.
Why it's wrong here
K-value mismatch prevents the neighbor from forming at all, not flapping with SIA.
✓
A unidirectional link issue is present, where EIGRP packets are successfully sent but not received, causing the query process to time out.
Why this is correct
Unidirectional link causes queries to be sent but replies not received, leading to SIA and neighbor flapping.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
✗
The EIGRP stub routing feature is enabled on one router, preventing query propagation and causing the active process to hang.
Why it's wrong here
Stub routing prevents queries from being sent to the stub router, but it does not cause SIA; it actually reduces the chance of SIA.
✗
The 'eigrp log-neighbor-changes' command is causing excessive logging, which delays EIGRP processing.
Why it's wrong here
Logging does not cause SIA or flapping; it only records events.

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?

VRF-Lite — This question tests VRF-Lite — OSPF neighbours must agree on key parameters..

What is the correct answer to this question?

The correct answer is: A unidirectional link issue is present, where EIGRP packets are successfully sent but not received, causing the query process to time out. — EIGRP uses the Reliable Transport Protocol (RTP) for updates, queries, and replies. If there is a unidirectional link issue (e.g., one direction has high latency or packet loss), the query process may not receive replies in time, causing the route to become SIA. This is a classic edge case where the link appears operational but is unidirectional for EIGRP packets.

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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