The answer is a short OSPF dead interval over the IPsec tunnel, which causes LSA aging out despite a current FULL neighbor state. This happens because the dead interval is too brief to accommodate the latency or jitter introduced by the IPsec encryption and decryption process, leading to periodic neighbor resets. When the adjacency temporarily drops, the receiving router ages the LSA to 3600 (MaxAge) and flushes it from the database, so even after the neighbor recovers to FULL, the stale LSA is gone and routes are not installed. On the CCNP ENARSI 300-410 exam, this scenario tests your understanding of how OSPF timers interact with tunnel interfaces—a common trap is assuming a FULL neighbor guarantees valid LSAs. Remember: a FULL neighbor only confirms the current hello exchange, not the integrity of previously learned LSAs. A useful memory tip is “Dead interval too tight, LSA takes flight”—if the dead timer is set too low for the IPsec overhead, LSAs will age out and vanish from the routing table.
300-410 IPsec Site-to-Site VPN Practice Question
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 are connected via an IPsec VPN tunnel. They are running OSPF over the tunnel. R1's show ip ospf neighbor shows R2 as FULL, but R1's show ip ospf database shows the LSA from R2 but with a high age (e.g., 3600). R1's show ip route does not have routes from R2. What is the root cause?
The OSPF dead interval on R1's tunnel interface is too short, causing frequent neighbor resets and LSA aging.
A short dead interval causes the neighbor to be declared dead and re-established, but LSAs are not refreshed, leading to high age and eventual removal.
B
The IPsec tunnel is dropping OSPF packets due to MTU issues.
Why wrong: MTU issues would cause packet loss, not LSA aging without neighbor flapping.
C
OSPF network type is point-to-multipoint, causing LSA flooding issues.
Why wrong: Point-to-multipoint works fine with IPsec tunnels.
D
R2 has a distribute-list out filtering OSPF routes.
Why wrong: A distribute-list would prevent LSA generation, but the LSA is present in the database.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The OSPF dead interval on R1's tunnel interface is too short, causing frequent neighbor resets and LSA aging.
The correct answer is A. When the OSPF dead interval on R1's tunnel interface is too short, R1 may temporarily lose neighbor adjacency with R2, causing the LSA from R2 to be prematurely aged out (age reaches 3600, the MaxAge) and removed from the OSPF database. Even though the neighbor state shows FULL at the moment of inspection, the LSA has already been flushed due to a previous timeout, so routes are not installed in the routing table.
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 dead interval on R1's tunnel interface is too short, causing frequent neighbor resets and LSA aging.
Why this is correct
A short dead interval causes the neighbor to be declared dead and re-established, but LSAs are not refreshed, leading to high age and eventual removal.
Related concept
Read the scenario before looking for a memorised answer.
✗
The IPsec tunnel is dropping OSPF packets due to MTU issues.
Why it's wrong here
MTU issues would cause packet loss, not LSA aging without neighbor flapping.
✗
OSPF network type is point-to-multipoint, causing LSA flooding issues.
Why it's wrong here
Point-to-multipoint works fine with IPsec tunnels.
✗
R2 has a distribute-list out filtering OSPF routes.
Why it's wrong here
A distribute-list would prevent LSA generation, but the LSA is present in the database.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between neighbor state (which can appear FULL momentarily after a flap) and LSA age (which reflects the last successful refresh), leading candidates to overlook that a short dead interval can cause LSA expiration without a permanent neighbor down state.
Detailed technical explanation
How to think about this question
OSPF LSAs have a MaxAge of 3600 seconds (1 hour); when a router does not receive a refresh from the originator within that time, it flushes the LSA. A short dead interval (e.g., 10 seconds) can cause transient neighbor flaps that disrupt LSA refresh cycles, leading to premature aging. In a real-world scenario, this often occurs when the IPsec tunnel experiences intermittent packet loss or delay, causing OSPF hello packets to be missed and the dead timer to expire, even if the tunnel eventually recovers.
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 dead interval on R1's tunnel interface is too short, causing frequent neighbor resets and LSA aging. — The correct answer is A. When the OSPF dead interval on R1's tunnel interface is too short, R1 may temporarily lose neighbor adjacency with R2, causing the LSA from R2 to be prematurely aged out (age reaches 3600, the MaxAge) and removed from the OSPF database. Even though the neighbor state shows FULL at the moment of inspection, the LSA has already been flushed due to a previous timeout, so routes are not installed in the routing table.
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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