- A
The tunnel MTU is set to 1500, but the GRE encapsulation adds 24 bytes, causing OSPFv3 packets to be fragmented.
Correct because the default tunnel MTU of 1500 does not account for GRE overhead, leading to fragmentation that OSPFv3 may not handle properly, especially with authentication or large LSAs.
- B
The OSPFv3 network type is set to point-to-point instead of broadcast.
Why wrong: Incorrect because both broadcast and point-to-point can form neighbors; the issue is MTU, not network type.
- C
The tunnel interface is missing the 'ipv6 ospf 1 area 0' command.
Why wrong: Incorrect because the engineer verified OSPFv3 is configured on both interfaces.
- D
The tunnel keepalive is misconfigured, causing the tunnel to flap.
Why wrong: Incorrect because the tunnel is up and pingable, so keepalive is not the issue.
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.
An engineer is troubleshooting a GRE IPv6 tunnel between two sites. The tunnel is up, and the engineer can ping the remote tunnel endpoint IPv6 address. However, OSPFv3 neighbors over the tunnel fail to form. The engineer verifies that OSPFv3 is configured on both tunnel interfaces with the same area and that the network type is broadcast. What is the most likely cause?
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 tunnel MTU is set to 1500, but the GRE encapsulation adds 24 bytes, causing OSPFv3 packets to be fragmented.
The GRE tunnel MTU of 1500 bytes does not account for the 24-byte GRE/IPv6 encapsulation overhead (20-byte IPv6 header + 4-byte GRE header). OSPFv3 packets, which can be up to 1500 bytes, become fragmented when encapsulated, but fragmentation is often disabled or handled poorly in tunnel interfaces, preventing OSPFv3 neighbor formation. This is the most likely cause because the tunnel is up and the endpoint is reachable, but the OSPFv3 packets are being dropped or corrupted due to fragmentation.
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 tunnel MTU is set to 1500, but the GRE encapsulation adds 24 bytes, causing OSPFv3 packets to be fragmented.
Why this is correct
Correct because the default tunnel MTU of 1500 does not account for GRE overhead, leading to fragmentation that OSPFv3 may not handle properly, especially with authentication or large LSAs.
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 OSPFv3 network type is set to point-to-point instead of broadcast.
Why it's wrong here
Incorrect because both broadcast and point-to-point can form neighbors; the issue is MTU, not network type.
- ✗
The tunnel interface is missing the 'ipv6 ospf 1 area 0' command.
Why it's wrong here
Incorrect because the engineer verified OSPFv3 is configured on both interfaces.
- ✗
The tunnel keepalive is misconfigured, causing the tunnel to flap.
Why it's wrong here
Incorrect because the tunnel is up and pingable, so keepalive is not the issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the concept that GRE encapsulation adds overhead, and candidates mistakenly think the tunnel being up and pingable means all traffic works, overlooking the MTU/fragmentation issue specific to OSPFv3 packets.
Detailed technical explanation
How to think about this question
GRE tunnels encapsulate IPv6 packets with an outer IPv4 header (20 bytes) and a GRE header (4 bytes), totaling 24 bytes of overhead. If the tunnel interface MTU is set to 1500, the effective MTU for the encapsulated payload is 1476 bytes, so OSPFv3 packets larger than 1476 bytes will be fragmented. Cisco routers by default set the tunnel interface MTU to 1476 bytes to avoid this, but if manually set to 1500, fragmentation can occur, and OSPFv3 does not handle fragmented packets well, leading to neighbor adjacency failure. A real-world scenario is when an engineer forgets to adjust the MTU after changing encapsulation types.
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 tunnel MTU is set to 1500, but the GRE encapsulation adds 24 bytes, causing OSPFv3 packets to be fragmented. — The GRE tunnel MTU of 1500 bytes does not account for the 24-byte GRE/IPv6 encapsulation overhead (20-byte IPv6 header + 4-byte GRE header). OSPFv3 packets, which can be up to 1500 bytes, become fragmented when encapsulated, but fragmentation is often disabled or handled poorly in tunnel interfaces, preventing OSPFv3 neighbor formation. This is the most likely cause because the tunnel is up and the endpoint is reachable, but the OSPFv3 packets are being dropped or corrupted due to fragmentation.
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
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