hardMultiple ChoiceObjective-mapped
300-410 Practice Question: Is troubleshooting an IPsec site-to-site VPN…
A network engineer is troubleshooting an IPsec site-to-site VPN where the tunnel is up, but the engineer notices that the 'show crypto ipsec sa' output shows that the number of packets encrypted is much higher than the number of packets decrypted on the remote side. What is the most likely cause?
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 remote router has a misconfigured route that sends return traffic out the wrong interface.
A significant mismatch between encrypted and decrypted packet counts suggests that some packets are being lost or dropped after encryption. The most common cause is a routing issue where the return traffic from the remote side is not taking the VPN tunnel, so the remote router does not decrypt those packets.
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 remote router has a misconfigured route that sends return traffic out the wrong interface.
Why this is correct
Correct because if the remote router does not have a route to the local LAN that points to the tunnel interface, the return traffic will be sent out the physical interface without encryption, and the local router will not see corresponding decrypted packets.
- ✗
The IPsec SA lifetime is set too low, causing frequent rekeying.
Why it's wrong here
Incorrect because frequent rekeying would cause brief interruptions but not a persistent mismatch in packet counts.
- ✗
The crypto map on the local router is applied to the wrong interface.
Why it's wrong here
Incorrect because if the crypto map were on the wrong interface, the tunnel would likely not be up.
- ✗
The access list in the crypto map on the remote router is too permissive, encrypting extra traffic.
Why it's wrong here
Incorrect because that would cause the remote router to encrypt more packets, not fewer.
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.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.