- A
The OSPF network type broadcast requires a DR/BDR election, but the tunnel is point-to-point, so the DR election fails and routes are not exchanged.
Why wrong: Broadcast network type can work on a tunnel if both routers are configured correctly; DR election will occur, but it does not prevent route exchange.
- B
The IPsec crypto map is configured to encrypt only unicast traffic, and OSPF hello packets are multicast (224.0.0.5), so they are dropped before encryption.
If the crypto ACL only permits unicast traffic (e.g., 'permit ip host A host B'), multicast OSPF packets are not matched and are sent in clear text or dropped, depending on the configuration. This is a common edge case where the interesting traffic definition does not include multicast.
- C
The OSPF hello and dead intervals are mismatched, preventing the neighbor from forming.
Why wrong: The question states the neighbor forms, so timers are not the issue.
- D
The IPsec tunnel is using transport mode, which does not support multicast traffic.
Why wrong: Transport mode can encapsulate multicast traffic if the tunnel is between two hosts; it is not a limitation.
300-410 IPsec Site-to-Site VPN Practice Question
This 300-410 practice question tests your understanding of ipsec site-to-site vpn. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 an IPsec site-to-site VPN between two routers using OSPF as the routing protocol. The OSPF neighbor forms, but routes are not being exchanged. The engineer verifies that the IPsec tunnel is up and that OSPF packets are being encrypted. The OSPF network type on the tunnel interface is set to broadcast. What is the most likely explanation for the missing routes?
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 crypto map is configured to encrypt only unicast traffic, and OSPF hello packets are multicast (224.0.0.5), so they are dropped before encryption.
Option B is correct because OSPF hello packets are sent to the multicast address 224.0.0.5, but IPsec crypto maps by default only encrypt unicast traffic. Since the crypto map does not match multicast packets, OSPF hellos are dropped before encryption, preventing OSPF neighbor adjacency from forming even though the IPsec tunnel is up and other packets are encrypted.
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 broadcast requires a DR/BDR election, but the tunnel is point-to-point, so the DR election fails and routes are not exchanged.
Why it's wrong here
Broadcast network type can work on a tunnel if both routers are configured correctly; DR election will occur, but it does not prevent route exchange.
- ✓
The IPsec crypto map is configured to encrypt only unicast traffic, and OSPF hello packets are multicast (224.0.0.5), so they are dropped before encryption.
Why this is correct
If the crypto ACL only permits unicast traffic (e.g., 'permit ip host A host B'), multicast OSPF packets are not matched and are sent in clear text or dropped, depending on the configuration. This is a common edge case where the interesting traffic definition does not include multicast.
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, preventing the neighbor from forming.
Why it's wrong here
The question states the neighbor forms, so timers are not the issue.
- ✗
The IPsec tunnel is using transport mode, which does not support multicast traffic.
Why it's wrong here
Transport mode can encapsulate multicast traffic if the tunnel is between two hosts; it is not a limitation.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that an IPsec tunnel being up guarantees all traffic is encrypted, but the trap here is that OSPF multicast packets are not matched by the default crypto ACL, causing OSPF to fail silently.
Detailed technical explanation
How to think about this question
Cisco crypto maps use access control lists (ACLs) to define which traffic to encrypt, and by default, ACLs do not match multicast addresses (224.0.0.0/4) unless explicitly configured. OSPF relies on multicast hellos (224.0.0.5) for neighbor discovery and keepalives; if these packets are not encrypted, they are sent in the clear or dropped, breaking OSPF adjacency. In real-world deployments, engineers often use 'ip ospf network point-to-multipoint' or configure a crypto ACL that includes multicast traffic to resolve this.
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 crypto map is configured to encrypt only unicast traffic, and OSPF hello packets are multicast (224.0.0.5), so they are dropped before encryption. — Option B is correct because OSPF hello packets are sent to the multicast address 224.0.0.5, but IPsec crypto maps by default only encrypt unicast traffic. Since the crypto map does not match multicast packets, OSPF hellos are dropped before encryption, preventing OSPF neighbor adjacency from forming even though the IPsec tunnel is up and other packets are encrypted.
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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