Question 1,951 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 protection using a transform set that includes ESP encryption and authentication. The tunnel comes up, but OSPFv3 over the tunnel fails to form adjacency. The engineer notices that the tunnel interface has an MTU of 1400. 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 IPsec transform set includes both ESP encryption and authentication, which adds 50+ bytes of overhead; the tunnel MTU of 1400 is too high for the actual path MTU after encapsulation.
With ESP encryption and authentication, the total overhead can be 50-60 bytes. The tunnel MTU of 1400 does not account for this, causing OSPFv3 packets to be fragmented or dropped.
B
OSPFv3 requires the tunnel interface to be configured with 'ipv6 ospf network point-to-point' to work over GRE.
Why wrong: OSPFv3 works over any interface type; point-to-point is not mandatory.
C
The IPsec configuration is missing the 'crypto map' applied to the tunnel interface.
Why wrong: If IPsec were not applied, the tunnel would still work; the issue is overhead.
D
The GRE tunnel mode should be 'tunnel mode gre ipv6' instead of the default.
Why wrong: The default GRE mode is ipv4; for IPv6 over GRE, 'tunnel mode gre ipv6' is correct, but the question implies it is configured.

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 IPsec transform set includes both ESP encryption and authentication, which adds 50+ bytes of overhead; the tunnel MTU of 1400 is too high for the actual path MTU after encapsulation.

The correct answer is A. When IPsec ESP encryption and authentication are applied to a GRE tunnel, the combined overhead (typically 50–60 bytes for ESP headers, trailers, and authentication data) reduces the effective payload MTU. With a tunnel interface MTU of 1400, the actual packet size after adding GRE (20 bytes) and IPsec overhead can exceed the path MTU, causing fragmentation or drops. OSPFv3 uses large hello packets (often 1500 bytes), and if the encapsulated packet exceeds the path MTU, adjacency cannot form.

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 IPsec transform set includes both ESP encryption and authentication, which adds 50+ bytes of overhead; the tunnel MTU of 1400 is too high for the actual path MTU after encapsulation.
Why this is correct
With ESP encryption and authentication, the total overhead can be 50-60 bytes. The tunnel MTU of 1400 does not account for this, causing OSPFv3 packets to be fragmented or dropped.
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 requires the tunnel interface to be configured with 'ipv6 ospf network point-to-point' to work over GRE.
Why it's wrong here
OSPFv3 works over any interface type; point-to-point is not mandatory.
✗
The IPsec configuration is missing the 'crypto map' applied to the tunnel interface.
Why it's wrong here
If IPsec were not applied, the tunnel would still work; the issue is overhead.
✗
The GRE tunnel mode should be 'tunnel mode gre ipv6' instead of the default.
Why it's wrong here
The default GRE mode is ipv4; for IPv6 over GRE, 'tunnel mode gre ipv6' is correct, but the question implies it is configured.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the concept that IPsec overhead must be accounted for when setting tunnel MTU, and candidates mistakenly assume that a tunnel MTU of 1400 is always safe for IPv6 over GRE with IPsec, ignoring the cumulative encapsulation overhead.

Detailed technical explanation

How to think about this question

The total overhead for IPv6 over IPv4 GRE with IPsec ESP (encryption + authentication) includes: 20 bytes (outer IPv4 header) + 4 bytes (GRE header) + 8–16 bytes (ESP IV/sequence) + variable padding + 12 bytes (ESP auth trailer) + 20 bytes (inner IPv6 header). This can easily exceed 50 bytes, meaning a 1400-byte tunnel MTU may still produce packets larger than the typical 1500-byte path MTU after encapsulation. OSPFv3 hello packets are 68 bytes plus IPv6 header, but the real issue is that the tunnel MTU is not adjusted downward to account for the IPsec overhead, leading to fragmentation or packet drops that prevent adjacency.

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.

Related 300-410 practice-question pages

Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.

Layer 3 Technologies practice questions

Practise 300-410 questions linked to Layer 3 Technologies.

EIGRP Troubleshooting practice questions

Practise 300-410 questions linked to EIGRP Troubleshooting.

OSPF Troubleshooting (v2/v3) practice questions

Practise 300-410 questions linked to OSPF Troubleshooting (v2/v3).

BGP Troubleshooting practice questions

Practise 300-410 questions linked to BGP Troubleshooting.

Route Redistribution practice questions

Practise 300-410 questions linked to Route Redistribution.

Policy-Based Routing (PBR) practice questions

Practise 300-410 questions linked to Policy-Based Routing (PBR).

VRF-Lite practice questions

Practise 300-410 questions linked to VRF-Lite.

Route Maps and Route Filtering practice questions

Practise 300-410 questions linked to Route Maps and Route Filtering.

Administrative Distance practice questions

Practise 300-410 questions linked to Administrative Distance.

Route Summarization practice questions

Practise 300-410 questions linked to Route Summarization.

Bidirectional Forwarding Detection (BFD) practice questions

Practise 300-410 questions linked to Bidirectional Forwarding Detection (BFD).

VPN Technologies practice questions

Practise 300-410 questions linked to VPN Technologies.

Practice this exam

Start a free 300-410 practice session

Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.

10 questions 20 questions 30 questions 50 questions Timed 30

300-410 practice-test guide →Study guide →Browse all practice tests

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 IPsec transform set includes both ESP encryption and authentication, which adds 50+ bytes of overhead; the tunnel MTU of 1400 is too high for the actual path MTU after encapsulation. — The correct answer is A. When IPsec ESP encryption and authentication are applied to a GRE tunnel, the combined overhead (typically 50–60 bytes for ESP headers, trailers, and authentication data) reduces the effective payload MTU. With a tunnel interface MTU of 1400, the actual packet size after adding GRE (20 bytes) and IPsec overhead can exceed the path MTU, causing fragmentation or drops. OSPFv3 uses large hello packets (often 1500 bytes), and if the encapsulated packet exceeds the path MTU, adjacency cannot form.

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.

About these practice questions

Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →

How Courseiva writes practice questions · Editorial policy

Keep practising

Question Discussion

Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.

Loading comments…

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.