- A
The ACL for interesting traffic is not mirrored on the remote router; it only permits traffic in one direction.
Correct. IPsec requires the ACL to be mirrored on both sides; otherwise, the traffic is not considered interesting for encryption.
- B
The transform-set uses AES-256, which is not supported on the remote router.
Why wrong: Incorrect. If the transform-set were unsupported, the tunnel would not come up at all.
- C
The 'crypto map' is applied to the wrong interface, such as the loopback instead of the physical interface.
Why wrong: Incorrect. If the crypto map were on the wrong interface, the tunnel might not come up, but the question states the tunnel is up.
- D
The 'ipsec-over-tcp' command is missing, causing the IPsec packets to be dropped.
Why wrong: Incorrect. IPsec-over-TCP is optional and not required for standard IPsec operation.
Quick Answer
The answer is that the ACL for interesting traffic is not mirrored on the remote router, permitting traffic in only one direction. This is the most likely explanation because IPsec requires the ACL on each peer to be a mirror image of the other—if the local ACL permits traffic from source to destination, the remote ACL must permit traffic from destination to source. Without this mirror, the tunnel can come up via IKE phase 1 and 2, but the router never classifies the traffic as interesting, so no encryption occurs even though the crypto map is correctly applied. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that a site-to-site VPN tunnel being up does not guarantee traffic encryption; the ACL mirror is a common trap where candidates assume a single-direction ACL is sufficient. A quick memory tip: "Tunnel up, no encrypt? Check the ACL mirror—one-way ACL means no crypto."
300-410 MPLS L3VPN Practice Question
This 300-410 practice question tests your understanding of mpls l3vpn. 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.
An engineer configures IPsec between two PE routers in an MPLS L3VPN using a site-to-site VPN. The engineer configures a transform-set with ESP encryption (AES-256) and ESP authentication (SHA-256). The engineer also configures an ACL to define interesting traffic. The VPN tunnel comes up, but no traffic is encrypted. The engineer verifies that the ACL is correct and that the crypto map is applied to the interface. 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 ACL for interesting traffic is not mirrored on the remote router; it only permits traffic in one direction.
A common edge case with IPsec is that the ACL for interesting traffic must be mirrored on both sides. If the ACL on one side permits traffic from source to destination, the other side must permit traffic from destination to source. If the ACLs are not mirrored, the tunnel may come up (due to IKE) but no traffic will be encrypted because the router does not consider the traffic as interesting. Another edge case is that the crypto map is applied to the wrong interface or that the ACL is using the wrong protocol (e.g., TCP instead of IP). However, the most likely explanation here is that the ACL is not correctly matching the traffic in both directions.
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 ACL for interesting traffic is not mirrored on the remote router; it only permits traffic in one direction.
Why this is correct
Correct. IPsec requires the ACL to be mirrored on both sides; otherwise, the traffic is not considered interesting for encryption.
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 uses AES-256, which is not supported on the remote router.
Why it's wrong here
Incorrect. If the transform-set were unsupported, the tunnel would not come up at all.
- ✗
The 'crypto map' is applied to the wrong interface, such as the loopback instead of the physical interface.
Why it's wrong here
Incorrect. If the crypto map were on the wrong interface, the tunnel might not come up, but the question states the tunnel is up.
- ✗
The 'ipsec-over-tcp' command is missing, causing the IPsec packets to be dropped.
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 L3VPN — This question tests MPLS L3VPN — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The ACL for interesting traffic is not mirrored on the remote router; it only permits traffic in one direction. — A common edge case with IPsec is that the ACL for interesting traffic must be mirrored on both sides. If the ACL on one side permits traffic from source to destination, the other side must permit traffic from destination to source. If the ACLs are not mirrored, the tunnel may come up (due to IKE) but no traffic will be encrypted because the router does not consider the traffic as interesting. Another edge case is that the crypto map is applied to the wrong interface or that the ACL is using the wrong protocol (e.g., TCP instead of IP). However, the most likely explanation here is that the ACL is not correctly matching the traffic in both directions.
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
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