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300-410 Practice Question: A dual-stack network using IPv6 over IPv4 GRE…
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?
⚠ Common exam trap
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.
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
✓
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.
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.
- ✗
IPv6 unicast-routing is not enabled on R1.
Why it's wrong here
OSPFv3 adjacency is up, so routing is enabled.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.