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Bidirectional Forwarding Detection (BFD) →hardMultiple ChoiceObjective-mapped

300-410 Bidirectional Forwarding Detection (BFD) Practice Question

This 300-410 practice question tests your understanding of bidirectional forwarding detection (bfd). 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 scenario where two routers running EIGRP are not forming an adjacency. Both routers have BFD configured under the EIGRP process and on the interfaces. The BFD session is up and operational. However, the EIGRP neighbor status shows 'Pending' and never transitions to 'Up'. 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.
Clue: "never"
Why it matters: Absolute qualifier. True only if the statement has zero exceptions — be cautious of options that seem obvious but break down in edge cases.

Question 1hardmultiple choice

Study the full EIGRP explanation →

A
The BFD timers are set too high, causing EIGRP to time out before BFD can respond.
Why wrong: BFD timers do not affect EIGRP's Pending state; the BFD session is already up.
B
EIGRP is configured with 'no auto-summary' on one router and 'auto-summary' on the other.
Why wrong: Auto-summary mismatch does not prevent adjacency formation; it affects route advertisement.
C
The EIGRP K-values are mismatched between the two routers.
EIGRP K-values must match for adjacency to form; a mismatch causes the neighbor to stay in Pending state even if BFD is up.
D
The interface is configured with 'bfd interval 50 min_rx 50 multiplier 3' but the neighbor expects different values.
Why wrong: BFD parameter negotiation allows different values; as long as the session is up, this is not the issue.

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 EIGRP K-values are mismatched between the two routers.

EIGRP requires the BFD session to be fully established before it will bring up the adjacency. If the BFD session is up but EIGRP is stuck in Pending, the issue is often a mismatch in EIGRP K-values or authentication.

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 BFD timers are set too high, causing EIGRP to time out before BFD can respond.
Why it's wrong here
BFD timers do not affect EIGRP's Pending state; the BFD session is already up.
✗
EIGRP is configured with 'no auto-summary' on one router and 'auto-summary' on the other.
Why it's wrong here
Auto-summary mismatch does not prevent adjacency formation; it affects route advertisement.
✓
The EIGRP K-values are mismatched between the two routers.
Why this is correct
EIGRP K-values must match for adjacency to form; a mismatch causes the neighbor to stay in Pending state even if BFD is up.
Clue confirmation
The clue words "most likely", "never" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
✗
The interface is configured with 'bfd interval 50 min_rx 50 multiplier 3' but the neighbor expects different values.
Why it's wrong here
BFD parameter negotiation allows different values; as long as the session is up, this is not the issue.

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?

Bidirectional Forwarding Detection (BFD) — This question tests Bidirectional Forwarding Detection (BFD) — OSPF neighbours must agree on key parameters..

What is the correct answer to this question?

The correct answer is: The EIGRP K-values are mismatched between the two routers. — EIGRP requires the BFD session to be fully established before it will bring up the adjacency. If the BFD session is up but EIGRP is stuck in Pending, the issue is often a mismatch in EIGRP K-values or authentication.

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", "never". 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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