300-410 Layer 3 Technologies Practice Question
A network engineer is configuring a GRE tunnel between two Cisco IOS XE routers, R1 and R2, to transport IPv6 traffic over an IPv4 network. The tunnel source is R1's GigabitEthernet0/0 interface (IPv4 address 10.1.1.1) and the tunnel destination is R2's GigabitEthernet0/0 interface (IPv4 address 10.2.2.2). The engineer configures the tunnel interface with IPv6 address 2001:DB8:1::1/64 and enables OSPFv3 on the tunnel interface. However, OSPFv3 adjacencies are not forming. What is the most likely cause?
⚠ Common exam trap
The trap here is overlooking the underlying transport reachability; the tunnel cannot come up if the IPv4 source and destination cannot communicate.
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 source and destination must be reachable via IPv4, and the tunnel interface must be up.
GRE tunnels require that the source and destination addresses are reachable via the transport network (IPv4 in this case). If the IPv4 network cannot route between the tunnel endpoints, the tunnel interface will be down, and no IPv6 traffic, including OSPFv3 hellos, will pass. The tunnel interface must be up and have the correct IPv6 address and OSPFv3 configuration.
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 must be configured with the `tunnel mode gre ipv6` command.
Why it's wrong here
The `tunnel mode gre ipv6` command is used for IPv6 over GRE, but in this scenario, the tunnel is transporting IPv6 over an IPv4 network. The correct tunnel mode for this is `tunnel mode gre ip`, which encapsulates IPv6 packets within IPv4. Using `tunnel mode gre ipv6` would attempt to encapsulate IPv6 within IPv6, which is not the case here.
- ✗
OSPFv3 requires the tunnel interface to be in the same area as the physical interfaces.
Why it's wrong here
OSPFv3 areas are independent of the physical interface configuration. The tunnel interface can be in a different area than the physical interfaces. The area ID is configured under the OSPFv3 process and applied to the interface. There is no requirement for the tunnel interface to be in the same area as the physical interfaces.
- ✗
The tunnel interface must have an IPv4 address configured for OSPFv3 to work.
Why it's wrong here
OSPFv3 runs directly over IPv6 and does not require an IPv4 address on the tunnel interface. The tunnel interface only needs an IPv6 address for OSPFv3 to form adjacencies. The underlying transport is IPv4, but that is handled by the tunnel encapsulation, not by an IPv4 address on the tunnel interface itself.
- ✓
The tunnel source and destination must be reachable via IPv4, and the tunnel interface must be up.
Why this is correct
For a GRE tunnel to be operational, the tunnel source and destination IPv4 addresses must be reachable. If the underlying IPv4 network cannot route between 10.1.1.1 and 10.2.2.2, the tunnel interface will remain down, and OSPFv3 will not form an adjacency. Additionally, the tunnel interface must be in an up/up state for OSPFv3 to run over it.
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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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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.