A router interface is configured for OSPF, but neighbors do not form. The engineer checks the interface and sees Hello 10 and Dead 40. The neighbor on the same segment uses Hello 30 and Dead 120.
What is the most likely cause of the OSPF adjacency failure?
Answer choices
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
Router IDs are identical
Duplicate router IDs can create OSPF problems, but the exhibit does not point to that issue. The detail the question emphasizes is the mismatch in hello and dead timer values. On exam-style troubleshooting questions, the best answer is usually the one directly supported by the data provided.
OSPF timers do not match
Correct. This is correct. OSPF neighbors must agree on timer values such as hello and dead intervals. If they do not, the devices will not form a full adjacency even if other parts of the link appear healthy.
The interface cost is too low
OSPF cost affects route preference after the adjacency has already formed. It does not normally stop the neighbor relationship from coming up in the first place. So even though cost is an OSPF concept, it is not the cause that best matches the symptoms shown here.
The subnet mask is invalid because it is a transit link
Transit links can absolutely use ordinary subnet masks, and the problem described is not about the mask choice. The key mismatch shown is the different OSPF timer configuration between the two routers.
Common exam trap
A frequent exam trap is selecting options related to router ID conflicts or interface cost as the cause of OSPF adjacency failure. Although router ID conflicts can disrupt OSPF operation, they typically cause instability rather than preventing neighbor formation outright. Similarly, interface cost influences route preference after adjacency is established and does not block neighbor relationships. The question’s emphasis on differing Hello and Dead timers is a direct clue that timer mismatch is the root cause. Misreading this detail leads to incorrect answers that overlook the fundamental OSPF requirement for matching timers to form adjacencies.
Technical deep dive
OSPF (Open Shortest Path First) is a link-state routing protocol that relies on neighbor relationships to exchange routing information. These neighbor relationships form through a process involving Hello packets, which are sent at regular intervals defined by the Hello timer. The Dead timer defines how long a router waits without receiving Hello packets before declaring a neighbor down. Both timers must match between neighbors on the same segment for adjacency to form successfully. OSPF requires that neighbors agree on several parameters, including Hello and Dead intervals, area ID, authentication, and subnet mask. If any of these parameters differ, the routers will not establish a full adjacency. The Hello timer controls how often Hello packets are sent, while the Dead timer determines the timeout period. A mismatch in these timers causes routers to expect different Hello packet intervals, preventing them from recognizing each other as neighbors. A common exam trap is confusing OSPF cost or router ID issues with adjacency failures caused by timer mismatches. While cost affects route selection after adjacency, and router ID conflicts can cause instability, the specific symptom of mismatched Hello and Dead timers directly prevents adjacency formation. In practical networks, this mismatch can cause interfaces to appear up with IP connectivity intact, yet OSPF neighbors never form, making timer synchronization critical for OSPF troubleshooting.
Related practice questions
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
Practise IPv4 subnetting, CIDR, masks, host ranges and subnet selection.
Practise OSPF neighbours, router IDs, metrics, areas and routing-table interpretation.
Practise VLANs, access ports, trunks, allowed VLANs and switching scenarios.
Practise spanning tree, root bridge election, port roles and STP troubleshooting.
Practise LACP, PAgP, port-channel behaviour and bundle requirements.
Practise standard and extended ACLs, permit/deny logic and traffic filtering.
Practise static NAT, dynamic NAT, PAT and inside/outside address translation.
Practise DHCP scopes, relay, leases and troubleshooting.
Practise routing-table output, longest-prefix match, AD and route selection.
Practise trunk verification and VLAN forwarding across switches.
Practise WLAN security, authentication and wireless architecture concepts.
Practise IPv6 addressing, routes, neighbour discovery and common IPv6 exam traps.
Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.
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Question 6
FAQ
OSPF neighbors must have matching Hello and Dead timer values to successfully form an adjacency on the same network segment.
The correct answer is: OSPF timers do not match — The adjacency fails because the OSPF hello and dead timers do not match. OSPF neighbors on the same segment must agree on several key parameters before they accept each other as neighbors, and the timer settings are one of those required matches. In plain terms, each router is expecting to hear OSPF hellos at one rhythm, but the other side is speaking at a different rhythm, so the relationship never becomes valid. This is different from cost, which matters later during path selection after neighbors are already exchanging information. The timer mismatch is a classic CCNA troubleshooting point because the interfaces can be up and IP connectivity can even appear normal while the OSPF adjacency still fails.
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
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