- A
The tunnel interface MTU must be explicitly set to 1500 to match the physical interface.
Why wrong: Setting tunnel MTU to 1500 would cause fragmentation of the outer IPv4 packet, which OSPFv3 does not handle well.
- B
The tunnel mode is incorrectly set to ipv6ip instead of gre; GRE adds additional overhead.
Why wrong: IPv6 manual tunnels use ipv6ip mode, which is correct; GRE is not required for IPv6 over IPv4.
- C
OSPFv3 uses the tunnel MTU (1460) for its packet size, but the physical interface MTU is 1500, causing the adjacency to stall.
OSPFv3 uses the interface MTU for database description packets. The tunnel MTU is 1460, but the physical path can only handle 1500 bytes including the outer header, leading to a mismatch.
- D
The tunnel source interface has a different OSPF area than the tunnel interface, preventing adjacency.
Why wrong: OSPF area configuration is on the interface, not the tunnel source; this would not cause EXSTART state.
Quick Answer
The answer is an MTU mismatch caused by the tunnel encapsulation overhead. OSPFv3 uses the tunnel interface’s default MTU of 1480 bytes—derived from the physical interface MTU of 1500 minus the 20-byte IPv4 header—to size its Database Description packets during the EXSTART state. When the physical interface MTU remains at 1500, the smaller tunnel MTU causes these DBD packets to be fragmented or silently dropped, stalling the adjacency indefinitely. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how IPv6 manual tunnels reduce the effective MTU and how OSPFv3’s DBD exchange is sensitive to packet size, often appearing as a trick where pings succeed but OSPF fails. A common trap is assuming a successful ping means the link is fully functional, but OSPFv3’s larger DBD packets reveal the mismatch. Memory tip: “Tunnel MTU shrinks by 20—OSPFv3 DBDs won’t flow if they’re too big to go.”
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 configures an IPv6 manual tunnel between two routers. The tunnel comes up and pings succeed, but OSPFv3 over the tunnel fails to form an adjacency, stuck in EXSTART state. MTU is set to 1500 on all physical interfaces. 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
OSPFv3 uses the tunnel MTU (1460) for its packet size, but the physical interface MTU is 1500, causing the adjacency to stall.
In an IPv6 manual tunnel (IPv6-in-IPv4), the tunnel interface has a default MTU of 1480 bytes (1500 minus 20-byte IPv4 header). OSPFv3 uses the tunnel MTU to determine the size of Database Description (DBD) packets during the EXSTART state. When the physical interface MTU is 1500, the tunnel MTU of 1480 causes OSPFv3 DBD packets to be fragmented or dropped, preventing the adjacency from progressing beyond EXSTART. Setting the tunnel MTU to 1480 or adjusting OSPFv3's ipv6 mtu-ignore can resolve this.
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 interface MTU must be explicitly set to 1500 to match the physical interface.
Why it's wrong here
Setting tunnel MTU to 1500 would cause fragmentation of the outer IPv4 packet, which OSPFv3 does not handle well.
- ✗
The tunnel mode is incorrectly set to ipv6ip instead of gre; GRE adds additional overhead.
Why it's wrong here
IPv6 manual tunnels use ipv6ip mode, which is correct; GRE is not required for IPv6 over IPv4.
- ✓
OSPFv3 uses the tunnel MTU (1460) for its packet size, but the physical interface MTU is 1500, causing the adjacency to stall.
Why this is correct
OSPFv3 uses the interface MTU for database description packets. The tunnel MTU is 1460, but the physical path can only handle 1500 bytes including the outer header, leading to a mismatch.
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 tunnel source interface has a different OSPF area than the tunnel interface, preventing adjacency.
Why it's wrong here
OSPF area configuration is on the interface, not the tunnel source; this would not cause EXSTART state.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the subtlety that OSPFv3 adjacency stalls in EXSTART due to MTU mismatch, where candidates mistakenly think the issue is with tunnel mode or OSPF area configuration rather than the default tunnel MTU reduction from encapsulation overhead.
Detailed technical explanation
How to think about this question
OSPFv3's EXSTART state involves routers exchanging DBD packets to negotiate the master/slave relationship and describe their LSDBs. The DBD packet size is derived from the interface MTU; if the tunnel MTU (1480) is smaller than the physical MTU (1500), the DBD packets may be silently dropped or cause fragmentation, stalling the process. In real-world scenarios, this is often resolved by configuring 'ipv6 ospf mtu-ignore' on the tunnel interface or manually setting the tunnel MTU to match the physical MTU minus the encapsulation overhead.
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.
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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: OSPFv3 uses the tunnel MTU (1460) for its packet size, but the physical interface MTU is 1500, causing the adjacency to stall. — In an IPv6 manual tunnel (IPv6-in-IPv4), the tunnel interface has a default MTU of 1480 bytes (1500 minus 20-byte IPv4 header). OSPFv3 uses the tunnel MTU to determine the size of Database Description (DBD) packets during the EXSTART state. When the physical interface MTU is 1500, the tunnel MTU of 1480 causes OSPFv3 DBD packets to be fragmented or dropped, preventing the adjacency from progressing beyond EXSTART. Setting the tunnel MTU to 1480 or adjusting OSPFv3's ipv6 mtu-ignore can resolve this.
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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Same concept, more angles
1 more ways this is tested on 300-410
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. 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?
hard- ✓ A.The tunnel MTU is set to 1500, but the GRE encapsulation adds 24 bytes, causing OSPFv3 packets to be fragmented.
- B.The OSPFv3 network type is set to point-to-point instead of broadcast.
- C.The tunnel interface is missing the 'ipv6 ospf 1 area 0' command.
- D.The tunnel keepalive is misconfigured, causing the tunnel to flap.
Why A: 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.
Last reviewed: Jun 24, 2026
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