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."
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 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)
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: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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
Static NAT maps one inside address to one outside address.
✗
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: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
Bidirectional Forwarding Detection (BFD) — This question tests Bidirectional Forwarding Detection (BFD) — Static NAT maps one inside address to one outside address..
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?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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Question Discussion
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