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 traffic from the remote LAN to the local LAN is not working. The engineer pings from the remote router to the local LAN IP and it succeeds. However, pings from a host on the remote LAN to a host on the local LAN fail. 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 local router does not have a route to the remote LAN subnet in its routing table.
The symptom indicates that the VPN tunnel is working for traffic sourced from the router itself, but not for traffic from the remote LAN. This is typically caused by a missing route on the local router for the remote LAN subnet, or a missing route on the remote router for the local LAN subnet, preventing the return traffic from being routed correctly.
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 map access list on the remote router does not include the remote LAN subnet.
Why it's wrong here
Incorrect because the tunnel is up and traffic from the remote router works, so the access list is likely correct.
- ✓
The local router does not have a route to the remote LAN subnet in its routing table.
Why this is correct
Correct because the local router must have a route to the remote LAN subnet to route the return traffic back through the tunnel. Without it, the return traffic is dropped.
- ✗
The IPsec transform set is missing the esp-sha-hmac authentication.
Why it's wrong here
Incorrect because the tunnel is up and working for router-initiated traffic, so the transform set is fine.
- ✗
The pre-shared key is mismatched between the two routers.
Why it's wrong here
Incorrect because a pre-shared key mismatch would prevent the tunnel from coming up at all.
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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Same concept, more angles
1 more way this is tested on 300-410
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A network engineer is troubleshooting an IPsec site-to-site VPN that uses a GRE tunnel. The GRE tunnel is up/up, and EIGRP is forming an adjacency over it. However, traffic from the local LAN to the remote LAN is not working. The engineer pings the remote LAN IP from the local router and it succeeds. What is the most likely cause?
medium- ✓ A.The local router does not have a route to the remote LAN subnet in its routing table.
- B.The crypto map access list does not include the local LAN subnet.
- C.The GRE tunnel keepalive is disabled.
- D.The IPsec transform set is missing authentication.
Why A: The GRE tunnel and routing protocol are working, but traffic from the LAN is failing. This indicates that the routing table on the local router does not have a route to the remote LAN subnet, or the route points to the wrong next-hop. The ping from the router succeeds because the router uses its own IP as source, which is directly connected to the tunnel.
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.