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.
This 350-601 practice question tests your understanding of network. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
An OSPF router in a broadcast network has not formed a neighbor relationship. What are three possible causes? (Choose three.)
Authentication incorrect
Why wrong: Also a possible cause, but not selected here.
MTU mismatch
Causes the routers to stay in ExStart state during database exchange.
Area ID mismatch
Why wrong: Also a possible cause, but not selected here.
Hello interval mismatch
Hello and dead timers must match for adjacency to form.
Network type mismatch
Different network types (e.g., broadcast vs point-to-point) can prevent adjacency.
Answer choices
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
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.
Related concept
Read the scenario before looking for a memorised answer.
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.
Related concept
Read the scenario before looking for a memorised answer.
Network type mismatch
Why this is correct
Different network types (e.g., broadcast vs point-to-point) can prevent adjacency.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
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.
Detailed technical explanation
OSPF uses the MTU value in the DBD packet's interface MTU field; if the receiving interface has a lower MTU, it will reject the DBD packet and the neighbor state will not progress past EXSTART. This is particularly relevant in mixed-vendor environments or when using jumbo frames inconsistently. The 'ip ospf mtu-ignore' command can bypass this check, but it is not recommended for production networks.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
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FAQ
Network — This question tests Network — Read the scenario before looking for a memorised answer..
The correct answer is: 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Same concept, more angles
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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?
mediumWhy 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.
Last reviewed: Jun 24, 2026
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