- A
The neighbor router is not configured for BFD.
Why wrong: While possible, the diagnostic code 1 specifically indicates that the control detection time expired, meaning packets were not received. This could be due to a connectivity issue or neighbor not sending.
- B
The BFD session is down because the local router is not sending BFD packets.
Why wrong: Tx Count is 50, so packets are being sent.
- C
The BFD session is down because the neighbor is not responding to BFD packets, likely due to a network connectivity issue or neighbor misconfiguration.
Local Diag 1 means control detection time expired, and Rx Count is 0, indicating no packets received from the neighbor. This points to a problem with the neighbor or the path.
- D
The BFD session is down because the multiplier is set too low.
Why wrong: The multiplier is 3, which is standard. The diagnostic code indicates a timeout, not a configuration issue.
Quick Answer
The answer is that the BFD session is down because the neighbor is not responding, as indicated by Local Diag 1, which means "Control Detection Time Expired." This diagnostic code tells you the local router stopped receiving BFD control packets from the neighbor within the configured detection interval, and the Rx Count of 0 confirms no packets were received despite 50 packets being sent. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your ability to interpret the show bfd neighbors detail output and differentiate between local and remote failures—a common trap is assuming a high Tx Count means the session is healthy, when in fact it only proves the local router is transmitting. For a quick memory tip, remember that Local Diag 1 equals "one-way traffic": you’re talking, but they’re not listening.
300-410 Bidirectional Forwarding Detection (BFD) Practice Question
This 300-410 practice question tests your understanding of bidirectional forwarding detection (bfd). Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 runs the following command on Router R1:
R1# show bfd neighbors detail
IPv4 Sessions NeighborAddr LD/RD Int State Holdown(mult) Intf
10.1.1.2 1/3 Gi0/0 Down 0(0) Gi0/0
Session state is DOWN OurAddr: 10.1.1.1 Handle: 1 Local Diag: 1, Demand mode: 0, Poll bit: 0 MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 3 Received MinRxInt: 1000000, Received Multiplier: 3 Holddown (hits): 0(0) Rx Count: 0, Tx Count: 50
Based on this output, what is the most likely cause of the BFD session being down?
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 session is down because the neighbor is not responding to BFD packets, likely due to a network connectivity issue or neighbor misconfiguration.
The Local Diag field shows 1, which indicates 'Control Detection Time Expired'. This means the local router did not receive BFD control packets from the neighbor within the expected time. The Rx Count is 0, confirming no packets were received. Tx Count is 50, so the local router is sending packets but not receiving any.
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 neighbor router is not configured for BFD.
Why it's wrong here
While possible, the diagnostic code 1 specifically indicates that the control detection time expired, meaning packets were not received. This could be due to a connectivity issue or neighbor not sending.
- ✗
The BFD session is down because the local router is not sending BFD packets.
Why it's wrong here
Tx Count is 50, so packets are being sent.
- ✓
The BFD session is down because the neighbor is not responding to BFD packets, likely due to a network connectivity issue or neighbor misconfiguration.
Why this is correct
Local Diag 1 means control detection time expired, and Rx Count is 0, indicating no packets received from the neighbor. This points to a problem with the neighbor or the path.
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 session is down because the multiplier is set too low.
Why it's wrong here
The multiplier is 3, which is standard. The diagnostic code indicates a timeout, not a configuration 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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Bidirectional Forwarding Detection (BFD) — study guide chapter
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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 session is down because the neighbor is not responding to BFD packets, likely due to a network connectivity issue or neighbor misconfiguration. — The Local Diag field shows 1, which indicates 'Control Detection Time Expired'. This means the local router did not receive BFD control packets from the neighbor within the expected time. The Rx Count is 0, confirming no packets were received. Tx Count is 50, so the local router is sending packets but not receiving any.
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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