Question 163 of 2,152

SNMP Troubleshooting →hardMultiple ChoiceObjective-mapped

Why IPsec Breaks SNMP: ACL Missing for SNMP Ports

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 polls the router successfully. The engineer then configures IPsec to encrypt all traffic between the router and the NMS. The NMS now fails to poll the router. 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 IPsec configuration does not include an ACL that matches SNMP traffic (UDP port 161).
If the IPsec crypto map's ACL does not match SNMP packets, the traffic is sent in clear text, but the NMS may expect encrypted traffic or the router may not process the packets correctly.
B
SNMPv3 encryption and IPsec encryption are incompatible and cannot be used together.
Why wrong: They can be used together, but it is redundant.
C
The IPsec configuration uses aggressive mode, which is incompatible with SNMPv3.
Why wrong: IPsec mode does not affect SNMPv3 compatibility.
D
The router's SNMP process must be restarted after IPsec is configured.
Why wrong: No restart is required.

Full breakdown with real-world context →

Quick Answer

The answer is a missing ACL for SNMP ports in the IPsec configuration. When IPsec is applied to encrypt all traffic between a router and an NMS, it relies on a crypto map that references an access list to define which traffic to encrypt. If that ACL does not explicitly match SNMP traffic on UDP port 161, the IPsec process will not encrypt or decrypt those packets, causing the NMS polls to fail even though SNMPv3 itself is configured correctly. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how IPsec interacts with management protocols and the common pitfall of assuming “all traffic” is automatically matched without a proper ACL. A frequent trap is forgetting that SNMPv3 already provides encryption, leading engineers to overlook the IPsec ACL requirement. Remember the mnemonic: “IPsec needs an invite—if SNMP’s not in the ACL, it won’t take flight.”

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 IPsec configuration does not include an ACL that matches SNMP traffic (UDP port 161).

When IPsec is configured to encrypt traffic between the router and the NMS, it must explicitly match the SNMP traffic using an access control list (ACL) that permits UDP port 161. Without this ACL, the IPsec policy does not recognize SNMP packets as interesting traffic and does not apply encryption, but more critically, the IPsec security association (SA) may not be established for SNMP packets, causing them to be dropped or not processed correctly. The NMS fails to poll because the SNMP requests are not being encrypted/decrypted as expected, breaking communication.

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 IPsec configuration does not include an ACL that matches SNMP traffic (UDP port 161).
Why this is correct
If the IPsec crypto map's ACL does not match SNMP packets, the traffic is sent in clear text, but the NMS may expect encrypted traffic or the router may not process the packets correctly.
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.
✗
SNMPv3 encryption and IPsec encryption are incompatible and cannot be used together.
Why it's wrong here
They can be used together, but it is redundant.
✗
The IPsec configuration uses aggressive mode, which is incompatible with SNMPv3.
Why it's wrong here
IPsec mode does not affect SNMPv3 compatibility.
✗
The router's SNMP process must be restarted after IPsec is configured.
Why it's wrong here
No restart is required.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that IPsec automatically encrypts all traffic on an interface, when in fact it requires an explicit ACL to match the desired traffic, and candidates may overlook the need to include SNMP (UDP 161) in that ACL.

Detailed technical explanation

How to think about this question

IPsec uses a crypto map or tunnel interface with an ACL to define interesting traffic; if the ACL does not include UDP port 161, SNMP packets are sent in the clear or are not processed by IPsec, potentially causing the NMS to receive unencrypted responses or no response if the router drops non-IPsec traffic. In real-world deployments, engineers often forget to extend the IPsec ACL when adding new services like SNMP, leading to silent failures that are hard to troubleshoot without packet captures.

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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.

Visual reference

Quick reference

VPN Protocol Comparison

Protocol	Port	Encryption	Authentication	Use Case
IKEv2 / IPsec	UDP 500 / 4500	AES-256	Certificates / PSK	Site-to-site & remote access
SSL / TLS VPN	TCP 443	TLS 1.3	Certificates / MFA	Clientless remote access
L2TP / IPsec	UDP 1701	AES (IPsec)	PSK / Certificates	Legacy remote access
WireGuard	UDP 51820	ChaCha20	Public keys	Modern high-performance VPN
PPTP	TCP 1723	MPPE (weak)	MS-CHAPv2	Legacy — avoid in production

PPTP is considered insecure. IKEv2/IPsec and SSL VPN are the current recommended options.

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 IPsec configuration does not include an ACL that matches SNMP traffic (UDP port 161). — When IPsec is configured to encrypt traffic between the router and the NMS, it must explicitly match the SNMP traffic using an access control list (ACL) that permits UDP port 161. Without this ACL, the IPsec policy does not recognize SNMP packets as interesting traffic and does not apply encryption, but more critically, the IPsec security association (SA) may not be established for SNMP packets, causing them to be dropped or not processed correctly. The NMS fails to poll because the SNMP requests are not being encrypted/decrypted as expected, breaking communication.

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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