300-410 Layer 3 Technologies Practice Question
A network engineer is configuring OSPFv3 on a dual-stack router. The router must form an adjacency with a neighbor over a link that supports both IPv4 and IPv6. The interface is configured with 'ipv6 ospf 1 area 0' and the router ID is manually set to 10.1.1.1. After applying the configuration, the engineer notices that the OSPFv3 adjacency remains in EXSTART state. What is the most likely cause?
⚠ Common exam trap
The trap here is assuming that EXSTART is caused by a router ID conflict or process ID mismatch, when it is typically an MTU mismatch.
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 IPv6 MTU on the interface does not match the neighbor's interface MTU.
The EXSTART state in OSPFv3 indicates that the routers have exchanged Hello packets and are attempting to negotiate the master/slave relationship for database description exchange. A mismatch in interface MTU prevents the successful exchange of database description packets, leaving the adjacency stuck in EXSTART. Ensuring consistent MTU on both sides resolves the issue.
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 router ID is not unique in the OSPFv3 domain.
Why it's wrong here
A duplicate router ID can cause adjacency issues, but EXSTART state typically indicates a mismatch in MTU or an issue with the master/slave negotiation during database exchange. A duplicate router ID would more likely result in the neighbor being stuck in INIT or EXSTART, but the scenario does not indicate duplication; the manual router ID is valid. The primary cause of EXSTART is MTU mismatch, not router ID duplication.
- ✗
The OSPFv3 process is not configured with an IPv6 address family.
Why it's wrong here
OSPFv3 does not use address families in the same way as OSPFv2; it natively supports IPv6. The command 'ipv6 ospf 1 area 0' enables OSPFv3 on the interface. If the address family were missing, the adjacency would not form at all, not remain in EXSTART. The EXSTART state indicates that Hello packets are being exchanged, so the process is active.
- ✗
The neighbor router is using a different OSPFv3 process ID.
Why it's wrong here
OSPFv3 process IDs are locally significant and do not need to match between neighbors. The adjacency can form even if process IDs differ. Therefore, a different process ID would not cause the adjacency to remain in EXSTART. The EXSTART state is related to database exchange parameters, such as MTU, not process ID.
- ✓
The IPv6 MTU on the interface does not match the neighbor's interface MTU.
Why this is correct
In OSPFv3, the EXSTART state is reached after the neighbor is seen in the Hello packet, but before database description packets are exchanged. A mismatch in interface MTU prevents the routers from agreeing on the initial database description sequence, causing the adjacency to remain in EXSTART. This is a common issue when IPv6 MTU is not consistent across the link, especially with tunneling or different link types.
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 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
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