The answer is a mismatched crypto ACL on the remote router, where the local router’s interesting traffic ACL permits the traffic, but the remote router’s ACL does not match the decrypted packets. This occurs because IPsec VPN traffic encapsulated no decapsulation means the local router is successfully encrypting and sending packets, but the remote peer either drops them upon receipt or cannot match them to its crypto map for decryption. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of symmetric ACL configuration across peers—a common trap is assuming the tunnel state alone guarantees traffic flow, when in fact the decryption process depends on the remote ACL mirroring the local ACL’s source and destination. A helpful memory tip: “Encapsulation is local, decapsulation is remote—if the remote ACL doesn’t mirror, the packet gets a tomb.”
300-410 Route Redistribution Practice Question
This 300-410 practice question tests your understanding of route redistribution. 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 establishes, but traffic does not pass. The 'show crypto ipsec sa' shows packets being encapsulated but no decapsulation. 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 on the local router permits traffic, but the remote router's crypto ACL does not match the decrypted traffic.
If packets are encapsulated but not decapsulated, the remote peer is likely not receiving the encrypted traffic or cannot decrypt it. A common edge case is that the interesting traffic ACL on one side does not match the actual traffic (e.g., mismatched source/destination), causing the remote peer to receive packets that do not match its crypto map ACL, so it drops them.
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 transform set on the remote peer is missing ESP encryption.
Why it's wrong here
Transform set mismatch would prevent SA establishment.
✓
The crypto ACL on the local router permits traffic, but the remote router's crypto ACL does not match the decrypted traffic.
Why this is correct
Mismatched interesting traffic ACLs cause the remote peer to not recognize the traffic as IPsec-protected.
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 IKE policy uses aggressive mode, which is incompatible with main mode.
Why it's wrong here
Mode mismatch prevents IKE phase 1, not SA after establishment.
✗
The 'crypto isakmp key' command uses a different pre-shared key on each side.
Why it's wrong here
Key mismatch would prevent IKE authentication.
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.
Route Redistribution — This question tests Route Redistribution — 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 on the local router permits traffic, but the remote router's crypto ACL does not match the decrypted traffic. — If packets are encapsulated but not decapsulated, the remote peer is likely not receiving the encrypted traffic or cannot decrypt it. A common edge case is that the interesting traffic ACL on one side does not match the actual traffic (e.g., mismatched source/destination), causing the remote peer to receive packets that do not match its crypto map ACL, so it drops them.
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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