This 300-410 practice question tests your understanding of ipsec site-to-site vpn. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Related concept
Read the scenario before looking for a memorised answer.
✗
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
OSPF uses a 16-bit metric field, allowing values from 1 to 65535, but the maximum installable cost is 65534; a cost of 65535 is treated as unreachable (infinite) and the route is not placed in the routing table. This is a common misconfiguration when administrators set the tunnel interface cost to the maximum value to discourage transit traffic, inadvertently preventing route installation. In real-world scenarios, this often occurs when using 'ip ospf cost 65535' on tunnel interfaces to prefer other paths, but forgetting that OSPF will not install the route at all.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
IPsec Site-to-Site VPN — This question tests IPsec Site-to-Site VPN — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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