hardMultiple Choice
300-410 Practice Question: An engineer configures an IPsec site-to-site VPN…
An engineer configures an IPsec site-to-site VPN between two routers. The tunnel comes up, but traffic is not encrypted. Which is the most likely explanation?
⚠ Common exam trap
Cisco often tests the misconception that a tunnel being up guarantees traffic encryption, but the crypto ACL is the gatekeeper for which traffic gets encrypted, and candidates may overlook this distinction.
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 does not match the traffic that is being sent.
The most likely reason traffic is not encrypted despite the tunnel being up is that the crypto ACL (access-list) applied to the IPsec configuration does not match the actual traffic being sent. The crypto ACL defines which traffic should be protected by IPsec; if the ACL does not permit the specific source/destination or protocol/port of the traffic, the router will not attempt to encrypt it, even though the IKE and IPsec security associations (SAs) are established.
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 ACL does not match the traffic that is being sent.
Why this is correct
Crypto ACLs define which traffic is protected by the IPsec transform set. If the ACL's permit entries do not match the actual source and destination subnets traversing the tunnel, packets are routed unencrypted via the crypto map's implicit deny, so the tunnel stays up while data flows in clear text.
- ✗
The transform set uses ESP with null encryption.
Why it's wrong here
ESP with null encryption authenticates packets but leaves payload plaintext, so the tunnel encrypts nothing. It is tempting because null encryption is a legitimate transform for integrity-only tunnels where confidentiality is not required, such as certain lab or compliance-testing setups.
- ✗
The ISAKMP policy has the wrong authentication method.
Why it's wrong here
A wrong ISAKMP authentication method (for example pre-shared key versus RSA signatures) prevents Phase 1 from completing, so the tunnel would not come up at all. It is tempting because authentication mismatches are a common IPsec fault, but they cause negotiation failure rather than plaintext traffic.
- ✗
The pre-shared key is incorrect.
Why it's wrong here
A mismatched pre-shared key prevents phase 1 or phase 2 completing, so the tunnel would not come up at all. It tempts because key errors are common IPsec faults, but here the tunnel is established while traffic passes unencrypted, pointing to a crypto ACL or interesting-traffic mismatch.
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.