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300-410 Practice Question: R1 and R2 have an IPsec VPN tunnel between their…
R1 and R2 have an IPsec VPN tunnel between their physical interfaces. They are running OSPF over the tunnel interface. R1's show ip ospf neighbor shows R2 as FULL, but R1's show ip route ospf does not include any routes from R2. R2's show ip route ospf shows routes from R1. What is the root cause?
⚠ Common exam trap
Cisco often tests the nuance that OSPF neighbor state FULL does not guarantee route installation; the trap here is that candidates assume a FULL adjacency means routes are automatically exchanged and installed, ignoring the OSPF cost limit of 65535.
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 OSPF cost on R1's tunnel interface is set to 65535, preventing route installation.
When the OSPF cost on R1's tunnel interface is set to 65535, OSPF considers the route unreachable because the maximum OSPF cost for route installation is 65534 (per RFC 2328). Routes with cost 65535 are not installed in the routing table, even though the neighbor state is FULL. This explains why R1 sees R2 as FULL but has no OSPF routes, while R2 (with a normal cost) installs routes from R1.
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 OSPF cost on R1's tunnel interface is set to 65535, preventing route installation.
Why this is correct
OSPF does not install routes with cost 65535 or higher (max metric). R1 receives LSAs but ignores them due to high cost.
- ✗
OSPF network type mismatch: R1 uses broadcast, R2 uses point-to-point.
Why it's wrong here
A mismatch would prevent adjacency from forming, but the neighbor is FULL.
- ✗
R1 has a distribute-list in filtering OSPF routes.
Why it's wrong here
A distribute-list could filter routes, but the question specifies no such configuration; the cost issue is more subtle.
- ✗
The IPsec tunnel is only encrypting unicast traffic, not OSPF multicast.
Why it's wrong here
If OSPF multicast were not encrypted, adjacency would not form; but it is FULL.
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.