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300-410 Practice Question: Configures OSPF on two routers connected via…
A network engineer configures OSPF on two routers connected via Ethernet. The adjacency forms but remains stuck in EXSTART state. Both routers have identical OSPF configuration except for MTU. Which is the most likely explanation?
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 MTU on the interfaces is mismatched, causing DBD packet rejection.
OSPF uses the interface MTU in Database Description (DBD) packets. If MTU values differ, the neighbor will reject DBD packets larger than its own MTU, causing the adjacency to stall in EXSTART. The fix is to use 'ip ospf mtu-ignore' or match MTU.
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 is point-to-point on one side and broadcast on the other.
Why it's wrong here
Network type mismatch typically causes adjacency to form but not exchange routes, not EXSTART stuck.
- ✓
The MTU on the interfaces is mismatched, causing DBD packet rejection.
Why this is correct
MTU mismatch is a classic cause of EXSTART state.
- ✗
The OSPF router ID is identical on both routers.
Why it's wrong here
Duplicate router ID prevents adjacency from forming at all.
- ✗
The 'ip ospf authentication' is configured only on one side.
Why it's wrong here
Authentication mismatch prevents neighbor state from progressing past INIT or EXSTART, but MTU is the most common cause.
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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