- A
The crypto map is not applied to the correct interface.
Correct because if the route to the remote LAN points to an interface without the crypto map, the router will attempt to send the packet unencrypted, resulting in encapsulation failure.
- B
The IPsec transform set is misconfigured with incompatible algorithms.
Why wrong: Incorrect because if the transform set were incompatible, the tunnel would not be up and SAs would not be established.
- C
The IKE keepalive timer is too short, causing frequent rekeying.
Why wrong: Incorrect because keepalive timers affect session stability but do not cause encapsulation failures.
- D
The MTU on the outside interface is too small, causing fragmentation.
Why wrong: Incorrect because fragmentation issues would typically show as 'pkts fragmented' or 'pkts not fragmented' counters, not encapsulation failures.
300-410 IPsec Site-to-Site VPN Practice Question
This 300-410 practice question tests your understanding of ipsec site-to-site vpn. 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.
A network engineer is troubleshooting an IPsec site-to-site VPN between two Cisco routers. The tunnel is up, but traffic intermittently drops. The engineer notices that the 'show crypto ipsec sa' output shows the packet counters incrementing for both encrypt and decrypt, but the 'pkts encaps failed' counter is also increasing. What is the most likely cause?
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 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 map is not applied to the correct interface.
The 'pkts encaps failed' counter indicates that the router is unable to encrypt packets that should be encrypted. This typically happens when the crypto map's access list matches traffic, but the route to the remote LAN points out an interface that does not have the crypto map applied, causing the router to try to send the packet without encryption.
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 crypto map is not applied to the correct interface.
Why this is correct
Correct because if the route to the remote LAN points to an interface without the crypto map, the router will attempt to send the packet unencrypted, resulting in encapsulation failure.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
- ✗
The IPsec transform set is misconfigured with incompatible algorithms.
Why it's wrong here
Incorrect because if the transform set were incompatible, the tunnel would not be up and SAs would not be established.
- ✗
The IKE keepalive timer is too short, causing frequent rekeying.
Why it's wrong here
Incorrect because keepalive timers affect session stability but do not cause encapsulation failures.
- ✗
The MTU on the outside interface is too small, causing fragmentation.
Why it's wrong here
Incorrect because fragmentation issues would typically show as 'pkts fragmented' or 'pkts not fragmented' counters, not encapsulation failures.
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.
Trap categories for this question
Command / output trap
Incorrect because fragmentation issues would typically show as 'pkts fragmented' or 'pkts not fragmented' counters, not encapsulation failures.
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 practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. 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 exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
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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IPsec Site-to-Site VPN — study guide chapter
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FAQ
Questions learners often ask
What does this 300-410 question test?
IPsec Site-to-Site VPN — This question tests IPsec Site-to-Site VPN — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: The crypto map is not applied to the correct interface. — The 'pkts encaps failed' counter indicates that the router is unable to encrypt packets that should be encrypted. This typically happens when the crypto map's access list matches traffic, but the route to the remote LAN points out an interface that does not have the crypto map applied, causing the router to try to send the packet without encryption.
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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