hardMultiple ChoiceObjective-mapped
300-410 Practice Question: An engineer is troubleshooting a GRE IPv6 tunnel…
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?
⚠ Common exam trap
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.
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.
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.
- ✗
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.
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
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