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300-410 Practice Question: An engineer configures OSPF on a link with an MTU…
An engineer configures OSPF on a link with an MTU of 1500 on one side and 1400 on the other. The OSPF neighbor state is stuck in EXSTART. What 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 router with the smaller MTU (1400) rejects DBD packets from the neighbor with MTU 1500 because they exceed the interface MTU, causing the EXSTART state.
OSPF uses the MTU of the interface to determine the maximum size of Database Description (DBD) packets. If the MTU mismatches, the router with the smaller MTU will reject DBD packets that exceed its MTU, causing the neighbor to remain in EXSTART.
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 with the smaller MTU (1400) rejects DBD packets from the neighbor with MTU 1500 because they exceed the interface MTU, causing the EXSTART state.
Why this is correct
OSPF DBD packets are sized based on MTU; a mismatch prevents the exchange from completing.
- ✗
The router with the larger MTU (1500) cannot process the smaller DBD packets from the neighbor.
Why it's wrong here
Smaller packets are not a problem; the issue is larger packets being fragmented or dropped.
- ✗
The OSPF network type is point-to-point, which ignores MTU mismatches.
Why it's wrong here
Point-to-point networks still use DBD packets and are affected by MTU mismatch.
- ✗
The 'ip ospf mtu-ignore' command is needed on both sides to bypass the MTU check.
Why it's wrong here
This command can fix the issue, but the question asks for the explanation of the problem.
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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