- A
The transform set on the remote peer is missing ESP encryption.
Why wrong: Transform set mismatch would prevent SA establishment.
- B
The crypto ACL on the local router permits traffic, but the remote router's crypto ACL does not match the decrypted traffic.
Mismatched interesting traffic ACLs cause the remote peer to not recognize the traffic as IPsec-protected.
- C
The IKE policy uses aggressive mode, which is incompatible with main mode.
Why wrong: Mode mismatch prevents IKE phase 1, not SA after establishment.
- D
The 'crypto isakmp key' command uses a different pre-shared key on each side.
Why wrong: Key mismatch would prevent IKE authentication.
Question 1,533 of 2,152
IPsec VPN: Packets Encapsulated but No Decapsulation
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.
Quick Answer
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.”
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 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: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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
Standard ACLs match source addresses.
- ✗
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: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
- Standard ACLs match source addresses.
- Extended ACLs can match source, destination, protocol and ports.
- The first matching ACL entry is used.
- There is usually an implicit deny at the end.
TExam Day Tips
- Check inbound versus outbound direction.
- Read the ACL from top to bottom.
- Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 300-410 ACL questions on filtering logic and placement.
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FAQ
Questions learners often ask
What does this 300-410 question test?
Route Redistribution — This question tests Route Redistribution — Standard ACLs match source addresses..
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 ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 300-410 ACL questions on filtering logic and placement.
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?
Standard ACLs match source addresses.
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Last reviewed: Jun 18, 2026
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