Quick Answer
The answer is matching authentication, timers, and area ID. These three conditions are required for OSPF routers to become fully adjacent because they directly govern the formation of the neighbor relationship and the synchronization of the link-state database. The area ID must match to ensure both routers share the same topological view; without it, they cannot exchange Type 1 LSAs and will stall before reaching the 2-Way state. Timers, specifically the Hello and Dead intervals, must be identical to maintain consistent keepalive communication, while authentication settings must align to validate routing updates and prevent unauthorized adjacencies. On the ENCOR 350-401 exam, this topic tests your understanding of OSPF neighbor state progression, and a common trap is assuming that matching router IDs or subnet masks are required—they are not for full adjacency, though subnet masks matter for the network type. A reliable memory tip is the acronym “A-T-A”: Area, Timers, Authentication—if any of these three differ, the adjacency will fail.
This 350-401 practice question tests your understanding of infrastructure. 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.
Which THREE conditions are required for OSPF routers to become fully adjacent? (Choose three.)
Same area ID
Routers must be in the same area.
Matching hello and dead intervals
Timers must match.
Matching authentication (if configured)
Authentication must match if used.
Different router IDs
Why wrong: Router IDs must be unique but not necessarily different? Actually they must be unique.
Same subnet mask
Why wrong: Subnet mask must match for broadcast links.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Same area ID
OSPF routers must share the same area ID to form a full adjacency because the area ID defines the link-state database scope. Routers in different areas cannot exchange Type 1 LSAs directly; they rely on ABRs for inter-area routing. Without matching area IDs, the routers will not even proceed to the 2-Way state.
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.
Same area ID
Why this is correct
Routers must be in the same area.
Related concept
Read the scenario before looking for a memorised answer.
Matching hello and dead intervals
Why this is correct
Timers must match.
Related concept
Read the scenario before looking for a memorised answer.
Matching authentication (if configured)
Why this is correct
Authentication must match if used.
Related concept
Read the scenario before looking for a memorised answer.
Different router IDs
Why it's wrong here
Router IDs must be unique but not necessarily different? Actually they must be unique.
Same subnet mask
Why it's wrong here
Subnet mask must match for broadcast links.
Common exam traps
Cisco often tests the misconception that OSPF requires matching subnet masks for adjacency, but the actual requirement is matching network addresses (subnet number), not the mask itself, except on broadcast networks where the mask affects DR/BDR election.
Detailed technical explanation
OSPF adjacency formation is governed by RFC 2328, where the Hello protocol checks for matching area ID, Hello/Dead intervals, and authentication (if configured) before moving to the 2-Way state. A subtle behavior is that on broadcast multi-access networks, a mismatched subnet mask will prevent the router from being elected as DR/BDR because the interface will be considered in a different subnet, but a full adjacency can still form between two routers if the network address matches. In real-world scenarios, misconfigured subnet masks on Ethernet links often cause partial adjacencies (2-Way/DROTHER) rather than full adjacency failure.
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.
What to study next
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FAQ
Infrastructure — This question tests Infrastructure — Read the scenario before looking for a memorised answer..
The correct answer is: Same area ID — OSPF routers must share the same area ID to form a full adjacency because the area ID defines the link-state database scope. Routers in different areas cannot exchange Type 1 LSAs directly; they rely on ABRs for inter-area routing. Without matching area IDs, the routers will not even proceed to the 2-Way state.
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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Last reviewed: Jun 11, 2026
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