Question 1,519 of 2,152

Bidirectional Forwarding Detection (BFD) →hardMultiple ChoiceObjective-mapped

Troubleshooting BFD Echo Failure from Debug

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 runs the following command to troubleshoot BFD session flapping:

R1# debug bfd packet

*Mar  1 00:15:23.456: BFD: [R1-to-R3] received async packet from 10.5.5.2, state UP, diag 0
*Mar  1 00:15:23.457: BFD: [R1-to-R3] sending async packet, state UP
*Mar  1 00:15:23.458: BFD: [R1-to-R3] received echo packet from 10.5.5.2, state UP
*Mar  1 00:15:23.459: BFD: [R1-to-R3] echo packet lost, no echo received for 300 ms
*Mar  1 00:15:23.460: BFD: [R1-to-R3] state UP -> DOWN (echo failure)

What does this output indicate?

A
BFD session is UP and stable.
Why wrong: The session transitions to DOWN due to echo failure.
B
BFD session went DOWN because of echo timeout, indicating possible path issue.
Echo packets were lost, causing BFD to declare the session down.
C
BFD async packets are failing, causing session down.
Why wrong: Async packets are being sent and received; only echo packets are lost.
D
BFD session is flapping due to misconfigured multiplier.
Why wrong: The cause is echo failure, not multiplier misconfiguration.

Full breakdown with real-world context →

Quick Answer

The answer is that the BFD session went DOWN because of an echo timeout, indicating a possible path issue. This debug output shows the router received an echo packet from 10.5.5.2 at 00:15:23.458, but then logged "echo packet lost, no echo received for 300 ms" just one millisecond later, triggering the state transition from UP to DOWN with the reason "echo failure." In BFD echo mode, the local router sends echo packets that the remote peer loops back; if these are not returned within the configured detection time (here, 300 ms), the session is torn down, which points to a unidirectional forwarding problem or misconfiguration of the echo source interface. On the CCNP ENARSI 300-410 exam, this scenario tests your ability to differentiate between async and echo failure causes—a common trap is confusing echo timeout with async hold-down timers. Remember the memory tip: "Echo failure = local loopback lost; async failure = remote hello missed."

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

✓

BFD session went DOWN because of echo timeout, indicating possible path issue.

The debug output shows that BFD detected an echo failure (no echo packets received for 300 ms), causing the session to go DOWN. This indicates a connectivity issue or misconfiguration affecting echo mode.

Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
BFD session is UP and stable.
Why it's wrong here
The session transitions to DOWN due to echo failure.
✓
BFD session went DOWN because of echo timeout, indicating possible path issue.
Why this is correct
Echo packets were lost, causing BFD to declare the session down.
Related concept
Read the scenario before looking for a memorised answer.
✗
BFD async packets are failing, causing session down.
Why it's wrong here
Async packets are being sent and received; only echo packets are lost.
✗
BFD session is flapping due to misconfigured multiplier.
Why it's wrong here
The cause is echo failure, not multiplier misconfiguration.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.

Detailed technical explanation

How to think about this question

This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.

KKey Concepts to Remember

Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
Use explanations to understand the rule behind the answer.

TExam Day Tips

Underline the problem statement mentally.
Watch for words such as best, first, most likely and least administrative effort.
Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

Real-world example

How this comes up in practice

A practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.

Visual reference

What to study next

Got this wrong? Here's your next step.

Identify which 300-410 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.

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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) — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: BFD session went DOWN because of echo timeout, indicating possible path issue. — The debug output shows that BFD detected an echo failure (no echo packets received for 300 ms), causing the session to go DOWN. This indicates a connectivity issue or misconfiguration affecting echo mode.

What should I do if I get this 300-410 question wrong?

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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