- A
The crypto map is applied to the wrong interface, and traffic is not being matched.
If the crypto map is applied to a different interface than the one carrying the interesting traffic, the traffic will not be encrypted even if the tunnel is up. The tunnel establishment only requires the crypto map to be present on the interface, but encryption only occurs for traffic matching the access-list on that specific interface.
- B
The 'transform-set' is configured with ESP-NULL, which provides no encryption.
Why wrong: ESP-NULL provides authentication but no encryption; however, the question states no traffic is encrypted, which could be true, but the tunnel would still be established. This is a possible but less likely cause compared to interface mismatch.
- C
The 'crypto isakmp key' is mismatched between the two routers.
Why wrong: A mismatched ISAKMP key would prevent the tunnel from establishing, not just encryption.
- D
The 'crypto map' is not applied globally, so it does not affect traffic.
Why wrong: Crypto maps are applied per interface, not globally. This statement is incorrect.
Quick Answer
The answer is that the crypto map is applied to the wrong interface, so traffic is not being matched for encryption. Even when an IPsec VPN tunnel is established, the crypto map must be applied to the specific interface through which the interesting traffic actually exits; if it is applied to a different interface—such as a subinterface while traffic flows through the main interface, or vice versa—the router will not attempt to encrypt that traffic, leaving it to pass in plaintext. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that tunnel establishment only proves IKE and IPsec negotiation succeeded, not that encryption is occurring on the correct data path. A common trap is assuming a working tunnel guarantees encryption, but the crypto map’s interface binding is what triggers the encryption engine. Remember the memory tip: “Tunnel up, traffic clear? Check the interface where the crypto map lives.”
300-410 MPLS Operations Practice Question
This 300-410 practice question tests your understanding of mpls operations. 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.
An engineer configures IPsec site-to-site VPN between two routers. The tunnel is established, but no traffic is encrypted. The engineer checks the crypto map and access-list and confirms they match the interesting traffic. What 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.
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 crypto map is applied to the wrong interface, and traffic is not being matched.
A common edge case is that the crypto map is applied to the wrong interface or the access-list is not correctly referencing the traffic. However, if the tunnel is established, the issue may be that the crypto map is applied to a subinterface but the traffic is flowing through the main interface, or the 'crypto map' command is missing the 'local-address' option for multiple crypto maps. Another possibility is that the 'set peer' command is missing or the peer address is incorrect.
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.
- ✓
The crypto map is applied to the wrong interface, and traffic is not being matched.
Why this is correct
If the crypto map is applied to a different interface than the one carrying the interesting traffic, the traffic will not be encrypted even if the tunnel is up. The tunnel establishment only requires the crypto map to be present on the interface, but encryption only occurs for traffic matching the access-list on that specific interface.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
- ✗
The 'transform-set' is configured with ESP-NULL, which provides no encryption.
Why it's wrong here
ESP-NULL provides authentication but no encryption; however, the question states no traffic is encrypted, which could be true, but the tunnel would still be established. This is a possible but less likely cause compared to interface mismatch.
- ✗
The 'crypto isakmp key' is mismatched between the two routers.
Why it's wrong here
A mismatched ISAKMP key would prevent the tunnel from establishing, not just encryption.
- ✗
The 'crypto map' is not applied globally, so it does not affect traffic.
Why it's wrong here
Crypto maps are applied per interface, not globally. This statement is incorrect.
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.
What to study next
Got this wrong? Here's your next step.
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.
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FAQ
Questions learners often ask
What does this 300-410 question test?
MPLS Operations — This question tests MPLS Operations — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The crypto map is applied to the wrong interface, and traffic is not being matched. — A common edge case is that the crypto map is applied to the wrong interface or the access-list is not correctly referencing the traffic. However, if the tunnel is established, the issue may be that the crypto map is applied to a subinterface but the traffic is flowing through the main interface, or the 'crypto map' command is missing the 'local-address' option for multiple crypto maps. Another possibility is that the 'set peer' command is missing or the peer address is incorrect.
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.
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?
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 18, 2026
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.
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