Question 730 of 2,152

Bidirectional Forwarding Detection (BFD) →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is an EIGRP K-value mismatch between the two routers. This is correct because EIGRP uses K values to calculate its composite metric, and the protocol mandates that these values must match exactly on both sides for an adjacency to form; if they differ, the neighbors will remain stuck in the INIT state even though BFD sessions are up and hello packets are being exchanged. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that BFD only accelerates link failure detection and does not enforce EIGRP’s neighbor negotiation rules, making it a common trap where engineers assume BFD health guarantees EIGRP adjacency. A key memory tip is “BFD for speed, K-values for agreement”—the mismatch prevents the handshake regardless of BFD status.

300-410 Bidirectional Forwarding Detection (BFD) Practice Question

This 300-410 practice question tests your understanding of bidirectional forwarding detection (bfd). This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 configures BFD for EIGRP on a point-to-point link. The BFD session is up, but EIGRP neighbors are stuck in INIT state. The engineer checks that EIGRP hello packets are sent and received. Which of the following 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, preventing the adjacency from forming.
Correct. EIGRP requires matching K values to form an adjacency. BFD does not influence this check.
B
The 'bfd all-interfaces' command is missing under the EIGRP process, so EIGRP ignores BFD state.
Why wrong: Incorrect. The 'bfd all-interfaces' command enables BFD integration, but the issue here is that EIGRP is stuck in INIT, not that BFD is not used.
C
The interface is configured with 'ip authentication mode eigrp' and 'ip authentication key-chain eigrp' with mismatched keys.
Why wrong: Incorrect. Authentication issues would cause EIGRP hello packets to be dropped, not stuck in INIT; the neighbor would not appear at all.
D
The EIGRP autonomous system number is different on each router.
Why wrong: Incorrect. A mismatched AS number would prevent the neighbor from being discovered; the router would not even see the neighbor.

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, preventing the adjacency from forming.

EIGRP requires that the K values match between neighbors. If the K values are mismatched, EIGRP will not form an adjacency even if BFD is operational. This is a common misconfiguration because BFD does not enforce K value matching; it only reports link failures.

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, preventing the adjacency from forming.
Why this is correct
Correct. EIGRP requires matching K values to form an adjacency. BFD does not influence this check.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
✗
The 'bfd all-interfaces' command is missing under the EIGRP process, so EIGRP ignores BFD state.
Why it's wrong here
Incorrect. The 'bfd all-interfaces' command enables BFD integration, but the issue here is that EIGRP is stuck in INIT, not that BFD is not used.
✗
The interface is configured with 'ip authentication mode eigrp' and 'ip authentication key-chain eigrp' with mismatched keys.
Why it's wrong here
Incorrect. Authentication issues would cause EIGRP hello packets to be dropped, not stuck in INIT; the neighbor would not appear at all.
✗
The EIGRP autonomous system number is different on each router.
Why it's wrong here
Incorrect. A mismatched AS number would prevent the neighbor from being discovered; the router would not even see the neighbor.

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

Command / output trap
Incorrect. The 'bfd all-interfaces' command enables BFD integration, but the issue here is that EIGRP is stuck in INIT, not that BFD is not used.

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, preventing the adjacency from forming. — EIGRP requires that the K values match between neighbors. If the K values are mismatched, EIGRP will not form an adjacency even if BFD is operational. This is a common misconfiguration because BFD does not enforce K value matching; it only reports link failures.

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 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?

hard

A.The BFD timers are set too high, causing EIGRP to time out before BFD can respond.
B.EIGRP is configured with 'no auto-summary' on one router and 'auto-summary' on the other.
✓ C.The EIGRP K-values are mismatched between the two routers.
D.The interface is configured with 'bfd interval 50 min_rx 50 multiplier 3' but the neighbor expects different values.

Why C: 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.

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