mediumMultiple ChoiceObjective-mapped
300-410 Practice Question: Is troubleshooting an IPsec site-to-site VPN that…
A network engineer is troubleshooting an IPsec site-to-site VPN that uses a GRE tunnel over IPsec. The GRE tunnel is up/up, but the routing protocol (EIGRP) running over the GRE tunnel is not forming an adjacency. The engineer checks the tunnel configuration and sees that the tunnel source and destination are correct. 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 crypto map access list does not match GRE protocol (47) traffic.
For a GRE over IPsec tunnel, the GRE tunnel must be protected by the crypto map. If the crypto map is applied to the physical interface but the GRE tunnel traffic is not matched by the crypto map's access list, the GRE packets will be sent unencrypted and the remote router will drop them, preventing the routing protocol from forming an adjacency.
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 does not match GRE protocol (47) traffic.
Why this is correct
Correct because GRE uses protocol 47; if the crypto map's access list only matches IP traffic between the LAN subnets, the GRE packets themselves are not encrypted and are dropped, causing the GRE tunnel to appear up but the routing protocol to fail.
- ✗
The EIGRP hello timer is set too high.
Why it's wrong here
Incorrect because a high hello timer would cause slow convergence, not a complete failure to form adjacency.
- ✗
The tunnel interface is not configured with an IP address.
Why it's wrong here
Incorrect because the tunnel is up/up, so it must have an IP address.
- ✗
The IPsec transform set does not include ESP encryption.
Why it's wrong here
Incorrect because the transform set is used for IPsec, but the issue is that GRE packets are not being encrypted at all.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
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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.