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300-410 Practice Question: Configures VRF-Lite with OSPF as the routing…
A network engineer configures VRF-Lite with OSPF as the routing protocol. Two routers in the same VRF are directly connected, but the OSPF neighbor state remains stuck in EXSTART/EXCHANGE. The engineer verifies that the MTU on both interfaces is 1500. Which 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 'ip mtu' command is configured on one interface with a value lower than 1500, causing the DBD packet to be larger than the receiving interface's IP MTU.
OSPF uses the IP MTU of the outgoing interface to set the size of Database Description (DBD) packets. If the receiving interface has a smaller IP MTU, the DBD packet is silently dropped, causing the neighbor to stay in EXSTART/EXCHANGE. Even if the physical MTU is 1500, the IP MTU can be lowered via 'ip mtu' command, and OSPF will use that lower value. The MTU mismatch is not necessarily the physical MTU but the IP MTU, which is a common edge case.
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 set to point-to-multipoint on one side and broadcast on the other, causing a mismatch in DBD packet size.
Why it's wrong here
Network type mismatch can cause adjacency issues, but it typically results in a state of DOWN or ATTEMPT, not EXSTART/EXCHANGE.
- ✓
The 'ip mtu' command is configured on one interface with a value lower than 1500, causing the DBD packet to be larger than the receiving interface's IP MTU.
Why this is correct
OSPF uses the IP MTU for DBD packets. If one interface has a lower IP MTU (e.g., 1400), the DBD packet from the other side (1500) is dropped, causing the stuck state.
- ✗
The VRF forwarding table is missing the OSPF route for the neighbor's router ID, preventing the exchange of DBD packets.
Why it's wrong here
OSPF adjacency is established at Layer 2; the VRF routing table does not affect the DBD exchange process.
- ✗
The OSPF dead interval is set to 40 seconds on one router and 120 seconds on the other, causing a mismatch in hello parameters.
Why it's wrong here
Hello/dead interval mismatch would prevent the neighbor state from progressing beyond INIT or 2WAY, not EXSTART/EXCHANGE.
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.
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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.