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300-410 Practice Question: Is troubleshooting an OSPFv3 adjacency issue…
A network engineer is troubleshooting an OSPFv3 adjacency issue between two routers over an IPv6-only link. The engineer configures OSPFv3 on both routers and sees that the adjacency is stuck in EXSTART state. 'debug ipv6 ospf packet' shows that the routers are exchanging DBD packets but not progressing. What is the most likely cause?
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 OSPFv3 interface MTU is mismatched between the two routers.
The EXSTART state indicates that the routers are trying to establish the master/slave relationship and exchange DBD packets. If they are stuck, it is often due to a mismatch in the OSPF MTU or the interface MTU. If the MTU is mismatched, the DBD packets may be too large to be sent, causing the routers to keep retransmitting. Another cause could be a mismatch in the OSPF network type, but that usually causes issues earlier. The most likely cause is an MTU mismatch.
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 OSPFv3 interface MTU is mismatched between the two routers.
Why this is correct
An MTU mismatch prevents DBD packets from being exchanged properly, causing the adjacency to be stuck in EXSTART.
- ✗
The routers have different OSPFv3 router IDs.
Why it's wrong here
Different router IDs are required for adjacency, so this would not cause an issue.
- ✗
The OSPFv3 process is configured with 'ipv6 ospf 1 area 0' on the interface, but the interface is not IPv6-enabled.
Why it's wrong here
If the interface were not IPv6-enabled, OSPFv3 would not even start the adjacency process.
- ✗
The link is a point-to-point link, but the OSPF network type is set to broadcast.
Why it's wrong here
A network type mismatch would cause the adjacency to be stuck in INIT or 2WAY, not EXSTART.
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
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