- A
The OSPF network type on the tunnel interface is set to non-broadcast, preventing DBD exchange.
Why wrong: Non-broadcast network type requires neighbor statements, but would not cause EXSTART state specifically; it would prevent adjacency formation entirely.
- B
The IPsec transform set uses ESP with authentication, adding 22 bytes of overhead, reducing the tunnel MTU to 1478 bytes, causing OSPF DBD packets larger than 1478 bytes to be dropped.
IPsec encapsulation adds overhead (e.g., 22 bytes for ESP-AES + SHA), reducing the effective MTU. OSPF DBD packets default to 1500 bytes on Ethernet, but if the tunnel MTU is lower, they are fragmented or dropped, leading to EXSTART state.
- C
The OSPF hello and dead intervals are mismatched between the two routers.
Why wrong: Mismatched timers would cause the neighbor to be stuck in INIT or DOWN state, not EXSTART.
- D
The IPsec tunnel is using transport mode instead of tunnel mode, corrupting OSPF packets.
Why wrong: Transport mode does not add a new IP header, but it still encapsulates OSPF packets; this would not typically cause EXSTART state.
300-410 IPsec Site-to-Site VPN Practice Question
This 300-410 practice question tests your understanding of ipsec site-to-site vpn. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
An engineer configures a site-to-site IPsec VPN between two routers using OSPF as the routing protocol. The OSPF neighbor becomes stuck in EXSTART state. The engineer verifies that the IPsec tunnel is up and that both routers can ping each other's tunnel interfaces. What is the most likely cause of the OSPF adjacency issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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 IPsec transform set uses ESP with authentication, adding 22 bytes of overhead, reducing the tunnel MTU to 1478 bytes, causing OSPF DBD packets larger than 1478 bytes to be dropped.
When OSPF neighbors are stuck in EXSTART state, it indicates a problem with Database Description (DBD) packet exchange. With an IPsec tunnel MTU of 1478 bytes (1500 minus 22 bytes for ESP authentication overhead), OSPF DBD packets that exceed this size are fragmented or dropped. Since IPsec does not support fragmentation of encrypted packets, the DBD exchange fails, preventing OSPF from progressing past EXSTART.
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 network type on the tunnel interface is set to non-broadcast, preventing DBD exchange.
Why it's wrong here
Non-broadcast network type requires neighbor statements, but would not cause EXSTART state specifically; it would prevent adjacency formation entirely.
- ✓
The IPsec transform set uses ESP with authentication, adding 22 bytes of overhead, reducing the tunnel MTU to 1478 bytes, causing OSPF DBD packets larger than 1478 bytes to be dropped.
Why this is correct
IPsec encapsulation adds overhead (e.g., 22 bytes for ESP-AES + SHA), reducing the effective MTU. OSPF DBD packets default to 1500 bytes on Ethernet, but if the tunnel MTU is lower, they are fragmented or dropped, leading to EXSTART state.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The OSPF hello and dead intervals are mismatched between the two routers.
Why it's wrong here
Mismatched timers would cause the neighbor to be stuck in INIT or DOWN state, not EXSTART.
- ✗
The IPsec tunnel is using transport mode instead of tunnel mode, corrupting OSPF packets.
Why it's wrong here
Transport mode does not add a new IP header, but it still encapsulates OSPF packets; this would not typically cause EXSTART state.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between OSPF states—candidates confuse EXSTART (DBD exchange failure) with other states like INIT (hello mismatch) or 2-WAY (neighbor discovery), and overlook the impact of IPsec overhead on MTU and packet fragmentation.
Detailed technical explanation
How to think about this question
OSPF DBD packets contain LSA headers and can be large, especially with many LSAs. The IPsec transform set with ESP authentication adds 22 bytes of overhead (16 bytes for HMAC-SHA1 or MD5, plus 6 bytes for SPI and sequence number), reducing the effective MTU to 1478 bytes. When a DBD packet exceeds this MTU, IPsec cannot fragment it because encryption occurs after fragmentation, causing the packet to be silently dropped. This is a common issue in DMVPN or site-to-site VPN designs where MTU is not adjusted via 'ip mtu' or 'ip tcp adjust-mss'.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this 300-410 question test?
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 IPsec transform set uses ESP with authentication, adding 22 bytes of overhead, reducing the tunnel MTU to 1478 bytes, causing OSPF DBD packets larger than 1478 bytes to be dropped. — When OSPF neighbors are stuck in EXSTART state, it indicates a problem with Database Description (DBD) packet exchange. With an IPsec tunnel MTU of 1478 bytes (1500 minus 22 bytes for ESP authentication overhead), OSPF DBD packets that exceed this size are fragmented or dropped. Since IPsec does not support fragmentation of encrypted packets, the DBD exchange fails, preventing OSPF from progressing past EXSTART.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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.
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