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 dual-stack network using IPv6 over IPv4 GRE tunnels with OSPFv3 is experiencing routing loops. Router R1 has the following relevant configuration: interface Tunnel0 ipv6 address 2001:DB8:1::1/64 tunnel source 192.0.2.1 tunnel destination 198.51.100.1 ipv6 ospf network point-to-point ipv6 ospf 1 area 0. Router R2 shows: R2# show ipv6 ospf neighbor Neighbor ID Pri State Dead Time Interface ID Interface 192.0.2.2 1 FULL/ - 00:00:33 7 Tunnel0. R2# show ipv6 route 2001:DB8:3::/64 O 2001:DB8:3::/64 [110/2] via 2001:DB8:1::1, Tunnel0. What is the root cause?
The OSPF network type on Tunnel0 should be broadcast to match the underlying GRE tunnel.
Why wrong: GRE tunnels are typically point-to-point; point-to-point is correct for OSPF.
B
The tunnel destination on R1 is incorrect, causing OSPF adjacencies to form with the wrong router.
Why wrong: The adjacency is formed correctly as shown.
C
R1 has a higher metric for the same prefix via another interface, causing a routing loop due to OSPF's SPF algorithm.
If R1 learns the prefix via a different path with a higher metric, it may still prefer the tunnel route, but if that tunnel route points back to R2, a loop occurs.
D
IPv6 unicast-routing is not enabled on R1.
Why wrong: OSPFv3 adjacency is up, so routing is enabled.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
R1 has a higher metric for the same prefix via another interface, causing a routing loop due to OSPF's SPF algorithm.
The routing loop is caused by R1 having a higher metric for the prefix 2001:DB8:3::/64 via another interface, which OSPF's SPF algorithm still installs in the routing table. Since R2's best path to that prefix points to R1 via Tunnel0 (metric 2), and R1's best path points back to R2 via a different interface with a higher metric, traffic bounces between them. This is a classic OSPF metric asymmetry issue in a dual-stack GRE tunnel environment.
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 on Tunnel0 should be broadcast to match the underlying GRE tunnel.
Why it's wrong here
GRE tunnels are typically point-to-point; point-to-point is correct for OSPF.
✗
The tunnel destination on R1 is incorrect, causing OSPF adjacencies to form with the wrong router.
Why it's wrong here
The adjacency is formed correctly as shown.
✓
R1 has a higher metric for the same prefix via another interface, causing a routing loop due to OSPF's SPF algorithm.
Why this is correct
If R1 learns the prefix via a different path with a higher metric, it may still prefer the tunnel route, but if that tunnel route points back to R2, a loop occurs.
Related concept
Read the scenario before looking for a memorised answer.
✗
IPv6 unicast-routing is not enabled on R1.
Why it's wrong here
OSPFv3 adjacency is up, so routing is enabled.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that routing loops in OSPF are caused by network type mismatches or tunnel misconfigurations, when the real issue is metric asymmetry between routers leading to recursive routing.
Trap categories for this question
Command / output trap
The adjacency is formed correctly as shown.
Detailed technical explanation
How to think about this question
OSPF's SPF algorithm computes the shortest path based on cumulative cost; if a router has two paths to the same prefix with different metrics, it installs the best (lowest cost) path. However, if the best path on R2 points to R1, and R1's best path points back to R2 via a higher-cost interface, a routing loop occurs because neither router has a direct path. In GRE tunnel environments, this often happens when the tunnel interface cost is misconfigured or when a physical interface has a lower metric than the tunnel, causing asymmetric routing.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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: R1 has a higher metric for the same prefix via another interface, causing a routing loop due to OSPF's SPF algorithm. — The routing loop is caused by R1 having a higher metric for the prefix 2001:DB8:3::/64 via another interface, which OSPF's SPF algorithm still installs in the routing table. Since R2's best path to that prefix points to R1 via Tunnel0 (metric 2), and R1's best path points back to R2 via a different interface with a higher metric, traffic bounces between them. This is a classic OSPF metric asymmetry issue in a dual-stack GRE tunnel environment.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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