300-410 VPN Technologies Practice Question
A network administrator is troubleshooting an IPsec site-to-site VPN between two Cisco routers. The VPN tunnel is up, but traffic from the local subnet to the remote subnet is not passing. The administrator checks the crypto ACL and finds that it matches the traffic. Which of the following is the most likely cause of the problem?
⚠ Common exam trap
The trap here is focusing solely on the crypto ACL and IPsec parameters while overlooking the need for a routing entry that directs traffic into the tunnel.
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
✓
There is no route for the remote subnet pointing to the tunnel interface or next-hop.
In an IPsec site-to-site VPN, the tunnel can be up due to traffic from other sources or because the peers established it, but for specific traffic to pass, the router must have a route to the remote subnet pointing to the tunnel or the next-hop. Without this route, packets are not sent through the tunnel and may be dropped or routed elsewhere. The crypto ACL matching is necessary but not sufficient; routing is equally important.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
There is no route for the remote subnet pointing to the tunnel interface or next-hop.
Why this is correct
Even if the IPsec tunnel is up, traffic will not pass if the router does not have a route to the remote subnet via the tunnel. The crypto ACL defines interesting traffic, but without a proper route, packets are either dropped or sent via the default route, not encrypted. This is a common oversight in site-to-site VPN configurations.
- ✗
The crypto map is applied to the wrong interface.
Why it's wrong here
If the crypto map were applied to the wrong interface, the VPN tunnel would not be up because the crypto map is responsible for establishing the tunnel. Since the tunnel is up, the crypto map is correctly applied to the outbound interface. The issue is likely related to routing or interesting traffic selection, not the crypto map application.
- ✗
The crypto ACL is configured with the wrong source and destination addresses.
Why it's wrong here
If the crypto ACL were incorrect, the tunnel might still be up if other traffic matched, but the specific traffic would not be encrypted. However, the administrator already checked that the crypto ACL matches the traffic. So this is not the cause. The issue is likely that the traffic is matched but not routed into the tunnel.
- ✗
The transform set is mismatched between peers.
Why it's wrong here
A mismatched transform set would prevent the IPsec tunnel from being established, as the peers must agree on encryption and authentication algorithms. Since the tunnel is up, the transform sets are compatible. The problem is not with the transform set but with something that allows the tunnel to be up while still blocking traffic, such as routing or ACL issues.
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 and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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.