350-501 Networking Practice Question
A network engineer is troubleshooting an OSPF adjacency failure between two directly connected routers, R1 and R2. Both routers are configured with the same OSPF process ID and area. The engineer verifies that the interfaces are up and IP connectivity exists. Which configuration mismatch is most likely causing the adjacency to fail?
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
✓
OSPF network type mismatch between the interfaces
An OSPF network type mismatch, such as one interface configured as point-to-point and the other as broadcast, prevents OSPF adjacency from forming. Options A, B, and C are incorrect: An MTU mismatch (A) usually allows adjacency to form but causes problems during LSA exchange; a passive interface (B) stops the router from sending hellos, so it wouldn't see the neighbor at all, but the question implies both interfaces are operational; an area ID mismatch (C) would prevent adjacency, but the scenario specifies that both routers are configured in the same area. Therefore, the most likely cause of the adjacency failure when interfaces are up and IP connectivity exists is a network type mismatch.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
MTU mismatch between the interfaces
Why it's wrong here
MTU mismatch causes OSPF packets to be silently dropped, but the adjacency may still reach the EXCHANGE state before failing.
- ✗
Passive interface configuration on one router
Why it's wrong here
Passive interface prevents sending hellos, so the neighbor would not be discovered at all.
- ✗
Area ID mismatch on the interfaces
Why it's wrong here
Area ID mismatch would result in a different area and the adjacency would not form due to area mismatch.
- ✓
OSPF network type mismatch between the interfaces
Why this is correct
Network type mismatch (e.g., broadcast vs. point-to-point) causes the routers to disagree on DR/BDR election and hello behavior, preventing adjacency.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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