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300-410 Practice Question: An engineer configures OSPF on two routers…
An engineer configures OSPF on two routers connected via a serial link. Both routers have 'ip ospf network point-to-point' configured, but the link is actually a Frame Relay multipoint subinterface. The OSPF neighbors remain stuck in EXSTART state. Which is the most likely explanation?
⚠ Common exam trap
Cisco often tests the distinction between neighbor states—specifically that EXSTART is reached only after hello and 2-WAY states are complete, so issues like mismatched area IDs or duplicate router IDs would prevent reaching EXSTART, while MTU mismatches specifically block DBD exchange at EXSTART.
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 serial link is not consistent between the two routers, causing DBD packets to be dropped.
When OSPF neighbors are stuck in EXSTART state, it typically indicates a problem with the Database Description (DBD) packet exchange. In this scenario, the 'ip ospf network point-to-point' command overrides the default Frame Relay multipoint behavior, but the actual link is a multipoint subinterface. This mismatch can cause MTU inconsistencies because the multipoint subinterface may have a different MTU than the point-to-point OSPF network type expects. If the MTU is not consistent between the two routers, DBD packets larger than the smaller MTU will be silently dropped, preventing the neighbors from progressing beyond 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 MTU on the serial link is not consistent between the two routers, causing DBD packets to be dropped.
Why this is correct
Correct. OSPF uses the interface MTU for DBD packets; mismatch prevents exchange.
- ✗
The OSPF hello timer is set too high, causing the neighbor to time out.
Why it's wrong here
Hello timer mismatch would cause a different state, not EXSTART.
- ✗
The area ID is different on the two routers.
Why it's wrong here
Area ID mismatch would prevent neighbor formation entirely, not stuck in EXSTART.
- ✗
The OSPF router ID is duplicated.
Why it's wrong here
Duplicate router ID would cause neighbor to flap, not stuck in 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
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.