- A
The SNMP engine ID on the router changed after the NMS was configured, causing authentication failures for traps.
The engine ID is used to derive keys; if it changes, the NMS cannot decrypt or authenticate traps, even though polling still works if the NMS uses the old engine ID.
- B
The 'snmp-server enable traps' command was omitted, so no traps are generated.
Why wrong: This would prevent all traps, not just SNMPv3 traps, and polling would still work.
- C
The NMS is using SNMPv2c for trap reception, which is incompatible with SNMPv3.
Why wrong: SNMPv3 traps can be received by an NMS that supports SNMPv3; the NMS must be configured for SNMPv3 traps.
- D
The 'snmp-server host' command specifies a community string instead of the SNMPv3 username.
Why wrong: The 'snmp-server host' command for SNMPv3 requires the username, not a community string; using a community string would cause a configuration error, but polling would also fail.
SNMPv3 Traps Not Received: Engine ID Change
This 300-410 practice question tests your understanding of snmp troubleshooting. 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 SNMPv3 on a router with the 'snmp-server group' and 'snmp-server user' commands, using SHA for authentication and AES for privacy. The NMS can poll the router successfully, but SNMP traps are not received. Which 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.
Quick Answer
The answer is that the SNMP engine ID on the router changed after the NMS was configured, causing authentication failures for traps. This occurs because SNMPv3 binds the user’s authentication and privacy keys to the engine ID at the time of configuration; if the engine ID changes—due to a router reload, configuration replacement, or manual deletion—the NMS will reject the trap as it cannot decrypt or validate the credentials. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that SNMPv3 traps rely on a separate notification view and that the engine ID must remain consistent between the agent and the NMS for trap delivery, even when polling succeeds. A common trap is assuming that successful polling guarantees trap functionality, but polling uses the read view while traps use the notify view, and both depend on a stable engine ID. Memory tip: “Polling works, traps fail? Check the engine ID—it’s the key that binds the trap.”
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 SNMP engine ID on the router changed after the NMS was configured, causing authentication failures for traps.
The most likely explanation is that the SNMP engine ID on the router changed after the NMS was configured, causing authentication failures for traps. SNMPv3 uses the engine ID as part of the key derivation for authentication and encryption. When the engine ID changes (e.g., after a configuration reload or a new crypto key generation), the NMS can still poll the router if it uses the old credentials for queries, but traps fail because the trap messages are authenticated with the new engine ID, which the NMS does not recognize.
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.
- ✓
The SNMP engine ID on the router changed after the NMS was configured, causing authentication failures for traps.
Why this is correct
The engine ID is used to derive keys; if it changes, the NMS cannot decrypt or authenticate traps, even though polling still works if the NMS uses the old engine ID.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The 'snmp-server enable traps' command was omitted, so no traps are generated.
Why it's wrong here
This would prevent all traps, not just SNMPv3 traps, and polling would still work.
- ✗
The NMS is using SNMPv2c for trap reception, which is incompatible with SNMPv3.
Why it's wrong here
SNMPv3 traps can be received by an NMS that supports SNMPv3; the NMS must be configured for SNMPv3 traps.
- ✗
The 'snmp-server host' command specifies a community string instead of the SNMPv3 username.
Why it's wrong here
The 'snmp-server host' command for SNMPv3 requires the username, not a community string; using a community string would cause a configuration error, but polling would also fail.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that SNMPv3 traps fail due to a missing 'enable traps' command or a version mismatch, but the real trap here is that the engine ID change breaks trap authentication while leaving polling functional, which is a subtle but critical distinction in SNMPv3 operation.
Trap categories for this question
Command / output trap
The 'snmp-server host' command for SNMPv3 requires the username, not a community string; using a community string would cause a configuration error, but polling would also fail.
Detailed technical explanation
How to think about this question
In SNMPv3, the engine ID is a unique identifier for the SNMP entity and is used to derive the authentication and privacy keys from the user's password via the HMAC-based key derivation algorithm (RFC 3414). If the engine ID changes (e.g., due to a router reload or a manual change via 'snmp-server engine-id local'), the keys derived for the user change, causing traps to fail authentication even though polling may still work if the NMS cached the old keys or uses a different authentication path. This is a common real-world issue when replacing hardware or after a configuration restore.
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.
TExam Day Tips
- 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.
Quick reference
Symmetric Encryption Algorithm Comparison
| Algorithm | Key Size | Block Size | Status | Notes |
|---|---|---|---|---|
| AES-128 | 128-bit | 128-bit | Current standard | NIST approved; WPA3, TLS |
| AES-256 | 256-bit | 128-bit | Current standard | Preferred for sensitive / govt data |
| 3DES | 112-bit effective | 64-bit | Deprecated (2023) | Replaced by AES |
| DES | 56-bit | 64-bit | Broken | Cracked in < 24 h; never deploy |
| ChaCha20 | 256-bit | Stream cipher | Current | TLS 1.3, WireGuard |
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this 300-410 question test?
SNMP Troubleshooting — This question tests SNMP Troubleshooting — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The SNMP engine ID on the router changed after the NMS was configured, causing authentication failures for traps. — The most likely explanation is that the SNMP engine ID on the router changed after the NMS was configured, causing authentication failures for traps. SNMPv3 uses the engine ID as part of the key derivation for authentication and encryption. When the engine ID changes (e.g., after a configuration reload or a new crypto key generation), the NMS can still poll the router if it uses the old credentials for queries, but traps fail because the trap messages are authenticated with the new engine ID, which the NMS does not recognize.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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