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300-410 Practice Question: An engineer configures MPLS on a router with OSPF…
An engineer configures MPLS on a router with OSPF as the IGP. The OSPF neighbor is stuck in EXSTART state. The engineer verifies that both routers have matching MTU values on the connecting interfaces. What is the most likely cause of this issue?
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 OSPF network type is mismatched between the two routers.
In OSPF, a neighbor stuck in EXSTART is often due to MTU mismatch, but if MTU is confirmed matching, the issue may be a mismatch in OSPF network type (e.g., one side broadcast, the other point-to-point). This causes different behavior in the Database Description (DD) packet exchange, leading to the stuck state even with matching 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 mismatched between the two routers.
Why this is correct
A mismatch in OSPF network type (e.g., broadcast vs point-to-point) can cause the neighbor to remain in EXSTART because the DD packet exchange process differs between network types, even if MTU is consistent.
- ✗
The OSPF hello and dead intervals are mismatched.
Why it's wrong here
Mismatched hello/dead intervals prevent the neighbor from reaching the 2-WAY state, not EXSTART. The neighbor would not progress past INIT.
- ✗
The router ID is duplicated on one of the routers.
Why it's wrong here
Duplicate router IDs cause OSPF to not form an adjacency at all, typically resulting in a state of DOWN or INIT, not EXSTART.
- ✗
The interface is configured with 'ip ospf mtu-ignore' on one side only.
Why it's wrong here
Using 'ip ospf mtu-ignore' on one side only can cause a mismatch in DD packet handling, but if MTU is already matching, this is unnecessary and not the primary 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.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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