- A
The EIGRP process is configured with 'bfd all-interfaces' but the interface is not configured with 'eigrp bfd'.
Why wrong: The 'bfd all-interfaces' command under EIGRP enables BFD on all interfaces that have EIGRP enabled, so this would not cause the issue.
- B
The BFD timers are configured with 'bfd interval 50 min_rx 50 multiplier 3' on one router and 'bfd interval 100 min_rx 100 multiplier 3' on the other, causing a negotiation failure.
BFD timers must be compatible; if the required interval or multiplier values are not negotiable, the session will fail to come up.
- C
The interface is configured with 'ip address 10.0.0.1 255.255.255.252' but the neighbor is using a different subnet.
Why wrong: A subnet mismatch would prevent EIGRP from forming, but BFD would still attempt to form; the BFD session would show 'Down' due to no response.
- D
The EIGRP autonomous system number is mismatched between the two routers.
Why wrong: AS number mismatch would prevent EIGRP adjacency, but BFD would still attempt to form; the BFD session would show 'Down' due to no response.
Quick Answer
The answer is a BFD timer mismatch, specifically when one router is configured with `bfd interval 50 min_rx 50 multiplier 3` and the other with `bfd interval 100 min_rx 100 multiplier 3`, causing the BFD session to remain down and preventing the EIGRP adjacency from forming. This occurs because EIGRP relies on an established Bidirectional Forwarding Detection session to signal neighbor reachability; if BFD timers do not match—particularly the transmit interval and minimum receive interval—the session fails to negotiate, leaving the BFD state as 'Down' even though the physical interface is up/up. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that BFD acts as an independent sub-layer for fast failure detection, and a common trap is assuming EIGRP hello or hold timers are the culprit when the real issue is a BFD parameter mismatch. Remember the memory tip: "BFD must agree to be seen—matching intervals keep the session green."
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 an EIGRP adjacency that is not forming. Both routers are configured for EIGRP with BFD. The engineer checks the BFD session and sees it is 'Down'. The interface between the routers is up/up. The EIGRP configuration appears correct. 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.
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 BFD timers are configured with 'bfd interval 50 min_rx 50 multiplier 3' on one router and 'bfd interval 100 min_rx 100 multiplier 3' on the other, causing a negotiation failure.
EIGRP BFD requires that the BFD session be established before the EIGRP adjacency can form. If the BFD session is down, the issue is often a mismatch in BFD parameters or a lack of BFD configuration on one side.
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 process is configured with 'bfd all-interfaces' but the interface is not configured with 'eigrp bfd'.
Why it's wrong here
The 'bfd all-interfaces' command under EIGRP enables BFD on all interfaces that have EIGRP enabled, so this would not cause the issue.
- ✓
The BFD timers are configured with 'bfd interval 50 min_rx 50 multiplier 3' on one router and 'bfd interval 100 min_rx 100 multiplier 3' on the other, causing a negotiation failure.
Why this is correct
BFD timers must be compatible; if the required interval or multiplier values are not negotiable, the session will fail to come up.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The interface is configured with 'ip address 10.0.0.1 255.255.255.252' but the neighbor is using a different subnet.
Why it's wrong here
A subnet mismatch would prevent EIGRP from forming, but BFD would still attempt to form; the BFD session would show 'Down' due to no response.
- ✗
The EIGRP autonomous system number is mismatched between the two routers.
Why it's wrong here
AS number mismatch would prevent EIGRP adjacency, but BFD would still attempt to form; the BFD session would show 'Down' due to no response.
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
The 'bfd all-interfaces' command under EIGRP enables BFD on all interfaces that have EIGRP enabled, so this would not cause the issue.
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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Bidirectional Forwarding Detection (BFD) — study guide chapter
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Bidirectional Forwarding Detection (BFD) practice questions
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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 BFD timers are configured with 'bfd interval 50 min_rx 50 multiplier 3' on one router and 'bfd interval 100 min_rx 100 multiplier 3' on the other, causing a negotiation failure. — EIGRP BFD requires that the BFD session be established before the EIGRP adjacency can form. If the BFD session is down, the issue is often a mismatch in BFD parameters or a lack of BFD configuration on one side.
What should I do if I get this 300-410 question wrong?
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.
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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Last reviewed: Jun 18, 2026
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