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300-410 Practice Question: An engineer configures BFD on an OSPF link…
An engineer configures BFD on an OSPF link between two routers. The BFD session comes up, but OSPF remains stuck in EXSTART state. The engineer verifies that IP connectivity is fine and OSPF hello packets are exchanged. 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
✓
An MTU mismatch between the two routers causes the DBD packets to be dropped, preventing OSPF from progressing past EXSTART.
BFD can cause OSPF to remain stuck in EXSTART if there is an MTU mismatch on the link. OSPF uses the interface MTU to determine the size of Database Description (DBD) packets. If the MTU is mismatched, the larger DBD packet is silently dropped by the receiving router, preventing the exchange of LSAs. BFD does not detect this because BFD packets are small and unaffected by MTU issues.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
An MTU mismatch between the two routers causes the DBD packets to be dropped, preventing OSPF from progressing past EXSTART.
Why this is correct
Correct. OSPF DBD packets are sized based on the interface MTU. A mismatch causes the larger DBD to be dropped, halting the adjacency while BFD remains up.
- ✗
BFD is not configured with the 'bfd all-interfaces' command under OSPF, causing OSPF to ignore BFD state changes.
Why it's wrong here
Incorrect. The 'bfd all-interfaces' command enables BFD for all OSPF interfaces, but the issue here is not BFD integration; it is an MTU mismatch.
- ✗
The OSPF network type is set to point-to-multipoint on one side and broadcast on the other, preventing the adjacency from forming.
Why it's wrong here
Incorrect. A network type mismatch would prevent the routers from reaching EXSTART at all; they would remain stuck in INIT or 2-WAY.
- ✗
The BFD minimum interval is set too low, causing BFD to flap and reset the OSPF adjacency repeatedly.
Why it's wrong here
Incorrect. A flapping BFD session would cause OSPF to reset the adjacency, not remain 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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JA
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.