Quick Answer
The answer is a duplicate router ID, which directly causes OSPF adjacency flapping because OSPF relies on the Router ID (RID) as the unique identifier for each router within the OSPF domain. When two routers, such as a Nexus 9000 spine and a leaf, share the same RID, they reject each other’s Hello packets, triggering repeated adjacency resets and preventing stable neighbor relationships. On the Cisco DCCOR / CCNP Data Center Core 350-601 exam, this scenario tests your understanding of OSPF operation in a spine-leaf fabric, where a misconfigured RID during a network upgrade can disrupt full-mesh convergence. A common trap is assuming the issue is a mismatched area or subnet, but the logs will show the adjacency flapping without authentication or MTU errors. Remember the mnemonic: “Two RIDs, one fight — OSPF takes flight.”
This 350-601 practice question tests your understanding of network. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
During a network upgrade, an engineer applies a new OSPF configuration on a Nexus 9000 spine. After the change, several leaf switches lose connectivity to each other. The engineer examines the logs and sees OSPF adjacency flapping. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Duplicate router IDs.
Duplicate router IDs cause OSPF neighbors to flap.
OSPF hello timer mismatch.
Why wrong: Hello timer mismatch would prevent adjacency from forming.
MTU mismatch on the fabric links.
Why wrong: MTU mismatch can cause problems but not typically flapping.
Incorrect area configuration.
Why wrong: Incorrect area would prevent adjacency entirely.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Duplicate router IDs.
Duplicate router IDs cause OSPF adjacency flapping because OSPF uses the Router ID (RID) to uniquely identify each router in the OSPF domain. When two routers share the same RID, they reject each other's Hello packets, leading to repeated adjacency resets. In a Nexus 9000 spine-leaf topology, this often occurs when the spine's RID is accidentally configured to match an existing leaf's RID, disrupting the entire fabric's OSPF convergence.
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.
Duplicate router IDs.
OSPF hello timer mismatch.
Why it's wrong here
Hello timer mismatch would prevent adjacency from forming.
MTU mismatch on the fabric links.
Why it's wrong here
MTU mismatch can cause problems but not typically flapping.
Incorrect area configuration.
Why it's wrong here
Incorrect area would prevent adjacency entirely.
Common exam traps
Cisco often tests the distinction between 'adjacency flapping' (caused by duplicate RIDs or mismatched authentication) and 'failure to form adjacency' (caused by hello/dead timer or MTU mismatches), so candidates mistakenly choose MTU or timer issues when the symptom is flapping rather than non-formation.
Detailed technical explanation
OSPF Router ID is a 32-bit value, typically derived from the highest loopback IP or manually set via 'router-id' command. When duplicates exist, the receiving router sees a Hello packet with its own RID and silently discards it (per RFC 2328, Section 9.2), causing the neighbor state to reset from FULL to DOWN. In a VXLAN/EVPN fabric with Nexus 9000, this is especially dangerous because OSPF is often used for underlay routing, and flapping adjacencies can cause BGP sessions to reset, leading to widespread reachability loss.
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
Network — This question tests Network — Read the scenario before looking for a memorised answer..
The correct answer is: Duplicate router IDs. — Duplicate router IDs cause OSPF adjacency flapping because OSPF uses the Router ID (RID) to uniquely identify each router in the OSPF domain. When two routers share the same RID, they reject each other's Hello packets, leading to repeated adjacency resets. In a Nexus 9000 spine-leaf topology, this often occurs when the spine's RID is accidentally configured to match an existing leaf's RID, disrupting the entire fabric's OSPF convergence.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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