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.
300-410 NAT and PAT Practice Question
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.
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.
IPsec encryption requires interesting traffic to trigger the tunnel. If the crypto ACL (access list) defining interesting traffic does not match the actual traffic (e.g., wrong source/destination or protocol), the traffic will be sent in the clear. Additionally, if the crypto map is not applied to the correct interface, encryption may not occur.
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 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
Static NAT maps one inside address to one outside address.
✗
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: 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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.
NAT and PAT — This question tests NAT and PAT — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The crypto ACL does not match the traffic that is being sent. — IPsec encryption requires interesting traffic to trigger the tunnel. If the crypto ACL (access list) defining interesting traffic does not match the actual traffic (e.g., wrong source/destination or protocol), the traffic will be sent in the clear. Additionally, if the crypto map is not applied to the correct interface, encryption may not occur.
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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