Question 88 of 2,152

IPv6 Tunneling Techniques →hardMultiple ChoiceObjective-mapped

300-410 IPv6 Tunneling Techniques Practice Question

This 300-410 practice question tests your understanding of ipv6 tunneling techniques. 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.

A network engineer configures an IPv6 over IPv4 GRE tunnel with IPsec using a crypto map. The tunnel works for unicast traffic, but OSPFv3 over the tunnel fails to form adjacency. The engineer checks the crypto map and sees that it only matches traffic with a specific access-list. What is the most likely explanation?

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.

Question 1hardmultiple choice

Review the full OSPF breakdown →

A
The crypto map access-list must include IPv6 protocol 89 (OSPF) to encrypt OSPFv3 packets; otherwise, they are sent in the clear and may be dropped.
OSPFv3 uses IPv6 protocol 89. If the access-list does not match this protocol, OSPFv3 packets are not protected and may be discarded by the remote IPsec policy.
B
OSPFv3 cannot be encrypted with IPsec; it requires a separate encryption mechanism.
Why wrong: IPsec can encrypt any IP protocol, including OSPFv3.
C
The GRE tunnel interface must be configured with 'tunnel protection ipsec' instead of a crypto map.
Why wrong: Both methods work; the issue is the access-list.
D
The crypto map is applied to the physical interface instead of the tunnel interface.
Why wrong: If the crypto map were applied to the physical interface, it would affect all traffic; the question implies it is applied correctly.

Full breakdown with real-world context →

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 crypto map access-list must include IPv6 protocol 89 (OSPF) to encrypt OSPFv3 packets; otherwise, they are sent in the clear and may be dropped.

OSPFv3 uses IPv6 protocol number 89 for its packets. When a crypto map is applied with an access-list that only matches specific traffic (e.g., unicast data), OSPFv3 packets (protocol 89) are not matched and are sent unencrypted. If the IPsec peer is configured to drop unencrypted traffic or if the GRE tunnel requires all traffic to be encrypted, OSPFv3 adjacency fails. Adding protocol 89 to the access-list ensures OSPFv3 packets are encrypted and processed correctly.

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 crypto map access-list must include IPv6 protocol 89 (OSPF) to encrypt OSPFv3 packets; otherwise, they are sent in the clear and may be dropped.
Why this is correct
OSPFv3 uses IPv6 protocol 89. If the access-list does not match this protocol, OSPFv3 packets are not protected and may be discarded by the remote IPsec policy.
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.
✗
OSPFv3 cannot be encrypted with IPsec; it requires a separate encryption mechanism.
Why it's wrong here
IPsec can encrypt any IP protocol, including OSPFv3.
✗
The GRE tunnel interface must be configured with 'tunnel protection ipsec' instead of a crypto map.
Why it's wrong here
Both methods work; the issue is the access-list.
✗
The crypto map is applied to the physical interface instead of the tunnel interface.
Why it's wrong here
If the crypto map were applied to the physical interface, it would affect all traffic; the question implies it is applied correctly.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that OSPFv3 cannot be encrypted with IPsec or that the crypto map must be applied to the tunnel interface, when the real issue is that the access-list used by the crypto map must explicitly include the OSPFv3 protocol (89) to encrypt routing updates.

Detailed technical explanation

How to think about this question

In a GRE over IPsec deployment, the crypto map access-list typically matches the GRE encapsulated packets (protocol 47) or the outer IP addresses. However, if the access-list is too restrictive (e.g., only matching specific ports or protocols), OSPFv3 packets (protocol 89) inside the GRE tunnel are not encrypted. This can cause the IPsec peer to drop the unencrypted packets if the security policy requires encryption for all traffic, or the OSPFv3 hello packets may be lost due to mismatched security associations. In real-world scenarios, this often manifests as OSPFv3 neighbors stuck in INIT state.

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?

IPv6 Tunneling Techniques — This question tests IPv6 Tunneling Techniques — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The crypto map access-list must include IPv6 protocol 89 (OSPF) to encrypt OSPFv3 packets; otherwise, they are sent in the clear and may be dropped. — OSPFv3 uses IPv6 protocol number 89 for its packets. When a crypto map is applied with an access-list that only matches specific traffic (e.g., unicast data), OSPFv3 packets (protocol 89) are not matched and are sent unencrypted. If the IPsec peer is configured to drop unencrypted traffic or if the GRE tunnel requires all traffic to be encrypted, OSPFv3 adjacency fails. Adding protocol 89 to the access-list ensures OSPFv3 packets are encrypted and processed correctly.

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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