The answer is a crypto ACL mirroring mismatch, where the access lists defining interesting traffic are not identical on both peers. For IPsec to establish the correct security associations (SAs), the ACL on each router must be a mirror image of the other, meaning the source and destination networks are swapped. If one router’s ACL permits traffic from LAN A to LAN B, the peer’s ACL must permit traffic from LAN B to LAN A; if they are both configured with the same source and destination, the SA negotiation fails for that traffic, leaving the tunnel up but data unencrypted. On the Cisco CCNP ENARSI 300-410 exam, this is a classic trap—candidates often verify the crypto map and interface but overlook that the ACL direction is reversed, causing a silent failure. A reliable memory tip is “mirror, not match”: the ACLs must reflect each other’s source and destination, not be identical.
300-410 Network Logging and Syslog Practice Question
This 300-410 practice question tests your understanding of network logging and syslog. 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 no traffic is encrypted. The engineer verifies that the crypto map is applied to the outgoing interface and that the ACL defining interesting traffic is correct. 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 is not mirrored on both sides, so the SA is not established for the traffic
A common edge case is that the ACL defining interesting traffic is applied in the wrong direction. For IPsec, the ACL should match traffic that will be encrypted, and it must be applied to the crypto map. However, if the ACL is configured with the source and destination reversed (e.g., source is local network and destination is remote network on one router, but on the other router the ACL should also have source as local network), the traffic may not match. Additionally, if the ACL is not mirrored on both sides, the tunnel may come up but traffic will not be encrypted because the SA is not established for that traffic.
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 is not mirrored on both sides, so the SA is not established for the traffic
Why this is correct
Correct. IPsec requires that the crypto ACLs be mirror images of each other for bidirectional traffic to be encrypted.
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 missing encryption or authentication
Why it's wrong here
A missing transform-set would prevent the tunnel from coming up at all.
✗
The crypto map is applied to the wrong interface
Why it's wrong here
If the crypto map is applied to the wrong interface, the tunnel would not come up.
✗
The IKE policy is not configured
Why it's wrong here
Without IKE policy, the tunnel would not establish.
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.
Network Logging and Syslog — This question tests Network Logging and Syslog — 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 is not mirrored on both sides, so the SA is not established for the traffic — A common edge case is that the ACL defining interesting traffic is applied in the wrong direction. For IPsec, the ACL should match traffic that will be encrypted, and it must be applied to the crypto map. However, if the ACL is configured with the source and destination reversed (e.g., source is local network and destination is remote network on one router, but on the other router the ACL should also have source as local network), the traffic may not match. Additionally, if the ACL is not mirrored on both sides, the tunnel may come up but traffic will not be encrypted because the SA is not established for that traffic.
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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