OSPF Neighbor Failure Causes – MTU, Hello Interval, Network Type
An OSPF router in a broadcast network has not formed a neighbor relationship. What are three possible causes? (Choose three.)
Quick Answer
The answer is an MTU mismatch, network type mismatch, and hello/dead interval mismatch. An MTU mismatch prevents OSPF neighbor formation because routers compare the MTU value inside Database Description packets; if the receiving interface’s MTU is smaller than the DBD packet, the packet is dropped, leaving the neighbor stuck in EXSTART/EXCHANGE. On the Cisco DCCOR and CCNP Data Center Core 350-601 exam, this question tests your ability to diagnose OSPF neighbor failure causes on broadcast networks, where a common trap is confusing MTU issues with authentication or area mismatches. Remember that OSPF uses DBD packets to negotiate the MTU, so a mismatch halts the exchange process entirely. For a quick memory tip, think “M-H-N”: MTU, Hello/Dead intervals, and Network type—these three must match for a neighbor to form.
⚠ Common exam trap
Cisco often tests the MTU mismatch as a subtle cause of OSPF neighbor failure, especially since it is less obvious than Hello/Dead interval or Area ID mismatches, and candidates may overlook it or confuse it with Layer 2 issues.
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
✓
MTU mismatch
In OSPF, an MTU mismatch prevents the formation of a neighbor relationship because OSPF routers compare the MTU value in Database Description (DBD) packets. If the receiving router's interface MTU is smaller than the DBD packet size, the packet is dropped, and the neighbor state remains stuck in EXSTART/EXCHANGE. This is a common issue on broadcast networks where different link types or misconfigured interfaces exist.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Authentication incorrect
Why it's wrong here
Also a possible cause, but not selected here.
- ✓
MTU mismatch
Why this is correct
Causes the routers to stay in ExStart state during database exchange.
- ✗
Area ID mismatch
Why it's wrong here
Also a possible cause, but not selected here.
- ✓
Hello interval mismatch
Why this is correct
Hello and dead timers must match for adjacency to form.
- ✓
Network type mismatch
Why this is correct
Different network types (e.g., broadcast vs point-to-point) can prevent adjacency.
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Same concept, more angles
1 more way this is tested on 350-601
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. An engineer is designing a Layer 3 network for a data center using OSPF. The core switches are connected to aggregation switches. To optimize convergence, which OSPF network type should be used on the links between core and aggregation?
medium- A.broadcast
- B.non-broadcast
- C.point-to-multipoint
- ✓ D.point-to-point
Why D: In a data center spine-leaf architecture, the links between core (spine) and aggregation (leaf) switches are typically point-to-point Layer 3 links. Configuring OSPF network type point-to-point (option D) on these interfaces eliminates the need for DR/BDR election, reduces hello and dead timers (default 10s/40s vs 30s/120s for broadcast), and allows faster convergence because OSPF immediately forms a neighbor adjacency without waiting for election delays.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-601 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 350-601 exam.