Question 539 of 2,152

SNMP Troubleshooting →hardMultiple ChoiceObjective-mapped

SNMPv3 Engine ID Change Causes Authentication Failure

This 300-410 practice question tests your understanding of snmp troubleshooting. 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 configures SNMPv3 with authentication and privacy on a router. The NMS can poll the router successfully. After a router reload, the NMS fails to poll the router, but the SNMP configuration is unchanged. 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.

A
The SNMP engine ID changed after the reload because it was not explicitly configured, causing authentication keys to be recalculated.
SNMPv3 keys are derived from the engine ID; if the engine ID changes, the NMS must be reconfigured with the new engine ID or the router must have a persistent engine ID configured.
B
The router lost its SNMP configuration due to a failed startup config.
Why wrong: The question states the configuration is unchanged, so the startup config is intact.
C
The NMS's SNMPv3 credentials were deleted during the reload.
Why wrong: The NMS is separate from the router and its configuration is not affected by the router reload.
D
The router's SNMP process failed to start after reload.
Why wrong: SNMP is typically integrated into the IOS and starts automatically; a failure would likely generate a syslog message.

Full breakdown with real-world context →

Quick Answer

The answer is that the SNMPv3 engine ID changed after the reload because it was not explicitly configured, causing the authentication keys to be recalculated and thus mismatching the NMS. This occurs because the SNMP engine ID is automatically derived from the router’s MAC address when not manually set; if the MAC address changes—even slightly, such as after a hardware swap or chassis reload—the engine ID changes, which forces the SNMPv3 authentication and privacy keys to be regenerated from the new ID. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that SNMPv3 security is tied to the engine ID, not just the configuration lines, and the common trap is assuming a reload preserves the engine ID. A reliable memory tip: “If you don’t set the ID, the reload will reset the key.”

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 changed after the reload because it was not explicitly configured, causing authentication keys to be recalculated.

The correct answer is A because SNMPv3 uses the engine ID as a seed to generate authentication and privacy keys. If the engine ID is not explicitly configured, it is automatically derived from the router's MAC address or other unique identifier. After a reload, the engine ID may change (e.g., due to a different interface coming up first), causing the NMS to recalculate keys that no longer match the router's keys, breaking authentication even though the configuration appears unchanged.

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 changed after the reload because it was not explicitly configured, causing authentication keys to be recalculated.
Why this is correct
SNMPv3 keys are derived from the engine ID; if the engine ID changes, the NMS must be reconfigured with the new engine ID or the router must have a persistent engine ID configured.
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 router lost its SNMP configuration due to a failed startup config.
Why it's wrong here
The question states the configuration is unchanged, so the startup config is intact.
✗
The NMS's SNMPv3 credentials were deleted during the reload.
Why it's wrong here
The NMS is separate from the router and its configuration is not affected by the router reload.
✗
The router's SNMP process failed to start after reload.
Why it's wrong here
SNMP is typically integrated into the IOS and starts automatically; a failure would likely generate a syslog message.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the subtle behavior that SNMPv3 keys are tied to the engine ID, and candidates mistakenly assume that unchanged SNMP configuration guarantees continued operation after a reload.

Detailed technical explanation

How to think about this question

Under the hood, SNMPv3 (RFC 3414) defines that the authentication and privacy keys are derived from a user's password and the SNMP engine ID using a key derivation algorithm (e.g., MD5 or SHA). If the engine ID changes, the derived keys change, causing authentication failures. In real-world scenarios, this often occurs when a router's SNMP engine ID is not explicitly set via the `snmp-server engine-id local` command, and the router's MAC address changes after a hardware swap or interface reordering.

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

Access Control Model Comparison

Model	Acronym	Who Controls Access?	Best For
Discretionary Access Control	DAC	Resource owner	Small teams, file shares
Mandatory Access Control	MAC	System / security labels	Classified govt / military
Role-Based Access Control	RBAC	Administrator (via roles)	Enterprise environments
Attribute-Based Access Control	ABAC	Policy engine (user + resource attributes)	Fine-grained, dynamic policies
Rule-Based Access Control	RuBAC	System rules / ACLs	Firewall rules, network ACLs

What to study next

Got this wrong? Here's your next step.

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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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 changed after the reload because it was not explicitly configured, causing authentication keys to be recalculated. — The correct answer is A because SNMPv3 uses the engine ID as a seed to generate authentication and privacy keys. If the engine ID is not explicitly configured, it is automatically derived from the router's MAC address or other unique identifier. After a reload, the engine ID may change (e.g., due to a different interface coming up first), causing the NMS to recalculate keys that no longer match the router's keys, breaking authentication even though the configuration appears unchanged.

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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