hardMultiple ChoiceObjective-mapped
300-410 Practice Question: An engineer configures OSPF between two PE…
An engineer configures OSPF between two PE routers in an MPLS L3VPN backbone. Both routers are directly connected via a GigabitEthernet link. The OSPF adjacency forms and then repeatedly flaps between FULL and EXSTART. The engineer verifies that both routers have matching OSPF network types (broadcast), matching area IDs, and no authentication mismatch. What is the most likely cause of the flapping?
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 one of the interfaces is lower than the other, causing DBD packet fragmentation.
In OSPF, if the MTU of the interface on one side is smaller than the size of a Database Description (DBD) packet, the neighbor will be stuck in EXSTART or the adjacency will flap. This is because OSPF uses the interface MTU to determine the maximum size of DBD packets, and if a router receives a DBD packet larger than its interface MTU, it drops it and the neighbor relationship cannot progress. This is a common edge case when one interface has a reduced MTU (e.g., due to MPLS overhead or misconfiguration) while the other uses the default 1500 bytes.
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 one of the interfaces is lower than the other, causing DBD packet fragmentation.
Why this is correct
Correct. A lower MTU on one side causes DBD packets to be dropped, preventing the adjacency from leaving EXSTART.
- ✗
The OSPF hello interval is mismatched between the two routers.
Why it's wrong here
Incorrect. A hello interval mismatch would prevent the adjacency from forming at all, not cause flapping between FULL and EXSTART.
- ✗
The OSPF dead interval is too short, causing the neighbor to be declared dead prematurely.
Why it's wrong here
Incorrect. A short dead interval would cause the neighbor to go DOWN, not to EXSTART.
- ✗
The OSPF router ID is duplicated on the segment.
Why it's wrong here
Incorrect. Duplicate router IDs would cause one router to reject the other, but the adjacency would not reach FULL and then flap to 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.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
MPLS Layer 3 VPN
A technology that uses Multiprotocol Label Switching to create secure, scalable virtual private networks that connect multiple sites at the network layer, where the service provider manages routing between customer sites.
Key term
LDP Protocol
LDP, or Label Distribution Protocol, is a protocol that routers use to automatically exchange labels that enable MPLS (Multiprotocol Label Switching) to create fast, efficient paths for data packets across a network.
About these practice questions
Courseiva writes every 300-410 question from scratch — 1,966 in total, each with an explanation and a wrong-answer breakdown. None are copied from real exams or dumps. Learn why practice questions differ from exam dumps →
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.