Why R1 and R2 OSPF Hello/Dead Timer Mismatch Blocks Adjacency
A network administrator is troubleshooting the OSPF configuration of routers R1 and R2. The routers cannot establish an adjacency relationship on their common Ethernet link. [sampleXML/xmlfile-321_1.png] The graphic shows the output of the show ip ospf interface e0 command for routers R1 and R2. Based on the information in the graphic, what is the cause of this problem?
Community Insight
The exam tests your knowledge that OSPF neighbors must agree on hello and dead timers to form an adjacency; the trap is that the same graphic also shows duplicate interface IP addresses, a defect no option covers, so you must pick the timer mismatch.
When two OSPFv2 routers cannot form an adjacency on a shared Ethernet link, the 'show ip ospf interface' output reveals which neighbor parameters must match. In this scenario, the hello and dead intervals on R1 and R2 differ, which is the listed cause of the failed adjacency.
Choosing A (improper OSPF area) or B (raise R1 priority): both routers are in area 0, and priority only influences DR election, not adjacency formation.
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Expert Analysis
Why the Answer Is Correct
Option D is correct because OSPF requires the hello and dead intervals to match between neighbors for a bidirectional adjacency to form. When the timers on R1 and R2's common Ethernet link disagree, the routers reject each other's hello packets and never progress beyond the INIT state. The 'show ip ospf interface' output in the graphic exposes this mismatch directly, and several community members — including arias and Portcullis — confirmed that "The hello and dead timers do not match" while rejecting option A.Why the Other Options Are Wrong
Option A is wrong because both interfaces display area 0, so the area is configured correctly and mismatched areas would instead appear as an area mismatch. Option B is wrong because priority affects only DR/BDR election and does not prevent adjacency; on a broadcast link with priority 0 both routers can still become neighbors. Option C is wrong because OSPF cost influences path selection, not neighbor establishment, and lowering or raising it has no bearing on adjacency. Option E is wrong because a BDR is optional and its absence does not block neighbor relationships. Option F is wrong because the OSPF process ID is a local-only value that need not match between routers.Community Comment Notes
Several learners flagged that the graphic also shows both e0 interfaces using the same IP address and both routers acting as DR — as Talloo put it, "the interfaces on both routers have the same IP address" — which is a genuine defect but is not among the answer choices. As cd1c33b complained, the item is "not accurate," and Micah7 noted that with the correct issue absent from the options testers "had to move on to another possibility." Among the available choices, the hello and dead timer mismatch (D) is the only technically valid OSPF adjacency blocker.Official Reference
Exam Strategy
Read the 'show ip ospf interface' output for the adjacency prerequisites checklist: area, network type, hello/dead timers, authentication, and subnet mask. Remember that a duplicate IP on both ends is never an option choice, so pick the mismatch that does appear in the options.
Frequently Asked Questions
Why is option D the best answer when both routers also share the same IP address?
A duplicate IP is a real fault visible in the output, but it is not offered as a choice. Among the listed options, only the hello and dead timer mismatch can be read from the output and blocks adjacency.
Do OSPF process IDs and priority need to match between neighbors?
No. The process ID is local to each router, and priority only decides DR/BDR election on a broadcast segment. Neither is required to match for an adjacency to form.
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