hardMultiple ChoiceObjective-mapped
300-410 Practice Question: An engineer configures IPsec site-to-site VPN…
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?
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.
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.
- ✗
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.
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.
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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.