- A
The crypto ACL does not match the traffic that is being sent.
Correct. Interesting traffic must match the ACL for encryption.
- B
The transform set uses ESP with null encryption.
Why wrong: Null encryption would still encrypt (with null algo), but traffic would be encapsulated.
- C
The ISAKMP policy has the wrong authentication method.
Why wrong: ISAKMP policy affects phase 1, not encryption of data.
- D
The pre-shared key is incorrect.
Why wrong: Incorrect PSK would prevent phase 1 from completing.
Question 1,914 of 2,152
IPsec VPN No Encryption: Crypto ACL Mismatch
This 300-410 practice question tests your understanding of nat and pat. 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 an IPsec site-to-site VPN between two routers. The tunnel comes up, but traffic is not encrypted. 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 a crypto ACL mismatch, meaning the access list defining interesting traffic does not match the actual traffic being sent. IPsec VPNs rely on a crypto ACL to identify which traffic should be encrypted; if the ACL specifies the wrong source, destination, or protocol, the router treats the traffic as uninteresting and forwards it in the clear, even though the tunnel is up. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of IPsec negotiation phases and the critical role of the crypto ACL as a traffic selector—a common trap is assuming a successful IKE phase 2 guarantees encryption. Remember the memory tip: “No match, no catch”—if the crypto ACL doesn’t catch the traffic, encryption won’t happen, so always verify the ACL entries against the actual data flow.
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 ACL does not match the traffic that is being sent.
The most likely reason traffic is not encrypted despite the tunnel being up is that the crypto ACL (access-list) applied to the IPsec configuration does not match the actual traffic being sent. The crypto ACL defines which traffic should be protected by IPsec; if the ACL does not permit the specific source/destination or protocol/port of the traffic, the router will not attempt to encrypt it, even though the IKE and IPsec security associations (SAs) are established.
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 crypto ACL does not match the traffic that is being sent.
Why this is correct
Correct. Interesting traffic must match the ACL for encryption.
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 transform set uses ESP with null encryption.
Why it's wrong here
Null encryption would still encrypt (with null algo), but traffic would be encapsulated.
- ✗
The ISAKMP policy has the wrong authentication method.
Why it's wrong here
ISAKMP policy affects phase 1, not encryption of data.
- ✗
The pre-shared key is incorrect.
Why it's wrong here
Incorrect PSK would prevent phase 1 from completing.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a tunnel being up guarantees traffic encryption, but the crypto ACL is the gatekeeper for which traffic gets encrypted, and candidates may overlook this distinction.
Detailed technical explanation
How to think about this question
IPsec uses a crypto ACL to classify traffic for encryption; this ACL is referenced in the crypto map and must match the interesting traffic (source, destination, protocol, port). Even if the tunnel is up, traffic that does not match the crypto ACL will be forwarded in clear text. A common real-world scenario is misconfigured ACLs that omit specific subnets or use wrong direction (e.g., inbound vs. outbound), leading to unencrypted traffic despite active SAs.
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.
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FAQ
Questions learners often ask
What does this 300-410 question test?
NAT and PAT — This question tests NAT and PAT — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The crypto ACL does not match the traffic that is being sent. — The most likely reason traffic is not encrypted despite the tunnel being up is that the crypto ACL (access-list) applied to the IPsec configuration does not match the actual traffic being sent. The crypto ACL defines which traffic should be protected by IPsec; if the ACL does not permit the specific source/destination or protocol/port of the traffic, the router will not attempt to encrypt it, even though the IKE and IPsec security associations (SAs) are established.
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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