OSPF Neighbor vs Adjacency Relationship Rules
Short answer
A Not all OSPF neighbors establish full adjacencies, particularly on multi-access networks where DROther routers remain in the 2-Way state.
Question
Key Takeaways
The main trap is confusing 'neighbor' status with 'adjacency' status; understanding DR/BDR election logic reveals that some neighbors intentionally stop at the 2-Way state.
Clarifies the distinction between OSPF neighbor relationships and full adjacencies, explaining why option A is the correct statement regarding OSPF behavior on multi-access networks.
Common mistake: Option C is often chosen incorrectly because candidates assume all OSPF peers must be fully adjacent to communicate, ignoring that Hello exchanges occur at the neighbor level without adjacency.
Expert Analysis
The correct answer is A.
In OSPF, neighbor discovery and adjacency formation are distinct phases governed by specific rules. When routers discover each other via Hello packets, they enter the 2-Way state. However, establishing a full Adjacency (Full state) is conditional. The key determinant is the router's role on the network segment: on broadcast and NBMA networks, Designated Routers (DR) and Backup Designated Routers (BDR) form adjacencies with all other routers, but non-DR/BDR routers (DROthers) only form adjacencies with the DR and BDR. Consequently, DROther routers do not become adjacent to each other, even though they remain neighbors in the 2-Way state. This confirms that not all neighbors establish adjacencies.
Option C is incorrect because adjacency is required for exchanging Link State Advertisements (LSAs), but routers can exchange basic information (like Hello packets) as neighbors without a full adjacency. Option B reverses the logical flow; neighbors are discovered first, and then adjacencies are formed selectively. Option D is incorrect because Database Description (DD) packets are used during the adjacency formation process to synchronize link-state databases, not to maintain the relationship once established.
Exam Strategy
When analyzing OSPF questions, always identify the network type (Broadcast, P2P, NBMA, or Point-to-Multipoint). If it is a multi-access network, check if DR/BDR election is implied. Remember that DROther routers never form adjacencies with each other, which immediately invalidates any assumption that all neighbors become adjacent.
Frequently Asked Questions
What state do non-DR/BDR routers reach when connecting to each other?
Can OSPF neighbors exchange routing information without being adjacent?
Community Votes
100% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 1 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 1
-
Stevemy 👍 1 Answered AEn OSPF, tous les voisins ne deviennent pas adjacents
Reader comments & corrections
No reader corrections yet — spotted an error or have a note? Share it below.
Related Analysis
Practice All H12-821 Questions
Access 168 questions with complete answers and detailed explanations.
View Full H12-821 Practice Test →