Question 159 of 2,011
hardMultiple ChoiceObjective-mapped
300-410 Practice Question: An engineer configures IPsec between two routers…
An engineer configures IPsec between two routers using a site-to-site VPN. The tunnel is established, but traffic is not encrypted. The engineer checks the crypto map and sees that the ACL for interesting traffic is configured correctly. Which is the most likely explanation?
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 ACL on the remote router does not mirror the local ACL, so the remote router does not initiate an SA for the return traffic.
A common edge case is that the ACL for interesting traffic is applied to the wrong interface or in the wrong direction. In site-to-site VPNs, the crypto map is applied to the outbound interface of the traffic. However, if the ACL is configured with the wrong source/destination (e.g., using the tunnel IP instead of the real IP), traffic will not match. Another less obvious issue is that the ACL must be symmetric; if the ACL on one router permits traffic from A to B, the other router must permit traffic from B to A. If one side is missing, the traffic may be sent but not encrypted because the other side does not have a matching SA.
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 ACL on the remote router does not mirror the local ACL, so the remote router does not initiate an SA for the return traffic.
Why this is correct
IPsec requires matching ACLs on both sides. If the remote ACL does not permit the return traffic, the SA will not be established for that direction, and traffic may be sent unencrypted.
- ✗
The crypto map is applied to the wrong interface (e.g., the inside interface instead of the outside interface).
Why it's wrong here
This would prevent encryption entirely, but the scenario says the tunnel is established, meaning the crypto map is likely applied correctly.
- ✗
The transform set uses ESP with authentication only, which does not provide encryption.
Why it's wrong here
ESP with authentication only (esp-md5-hmac) does not encrypt, but the scenario says the tunnel is established, and the engineer expects encryption. This could be a valid issue, but the question focuses on ACL mirroring.
- ✗
The IKE policy uses aggressive mode, which does not support encryption.
Why it's wrong here
Aggressive mode is a phase 1 negotiation method and does not affect encryption; it only affects identity protection.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Last reviewed: Jun 19, 2026
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.