The correct answer is to increase the OSPF cost on the leaf-to-spine links. This resolves OSPF underlay instability after adding a new spine switch because the new device advertises routes with a lower cost, causing abrupt traffic shifts and route flapping as leaf switches constantly recalculate paths. By raising the cost on these links, you make the new spine less preferred, allowing the routing table to stabilize through gradual convergence rather than sudden, disruptive changes. On the Cisco DCCOR / CCNP Data Center Core 350-601 exam, this scenario tests your understanding of OSPF metric manipulation in VXLAN EVPN fabrics—a common trap is to assume the issue is a misconfiguration of the new spine’s OSPF process or a missing neighbor relationship, but the real culprit is cost asymmetry. Remember: when a new spine enters the fabric, think “cost control, not protocol restart.” A quick memory tip is “New spine, cost incline”—increasing the cost on leaf-to-spine links keeps the underlay stable.
350-601 Network Practice Question
This 350-601 practice question tests your understanding of network. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
After adding a new spine switch to a VXLAN EVPN fabric with OSPF underlay, some leaf switches experience routing instability. Which action could resolve the instability?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Increase the OSPF cost on the leaf-to-spine links.
When a new spine switch is added to a VXLAN EVPN fabric with an OSPF underlay, the leaf switches may experience routing instability because the new spine advertises routes with a lower cost, causing traffic to shift abruptly. Increasing the OSPF cost on the leaf-to-spine links makes those paths less preferred, stabilizing the routing table by preventing flapping and ensuring a more gradual convergence.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Increase the OSPF cost on the leaf-to-spine links.
Why this is correct
Higher cost makes the new spine less preferred, stabilizing routing.
Related concept
Read the scenario before looking for a memorised answer.
✗
Configure OSPF neighbor authentication.
Why it's wrong here
Authentication adds security but does not affect stability from equal-cost paths.
✗
Decrease the OSPF hello timer.
Why it's wrong here
Decreasing hello timer may increase load and worsen instability.
✗
Enable OSPF route summarization on the leaves.
Why it's wrong here
Summarization reduces route size but does not prevent flap from equal-cost paths.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that routing instability is caused by security or timer issues, when in fact it is typically due to unequal cost paths causing SPF thrashing after a new device is added.
Detailed technical explanation
How to think about this question
In OSPF, the cost metric is inversely proportional to bandwidth (default cost = 10^8 / bandwidth), and adding a new spine can introduce a lower-cost path, triggering SPF recalculations and route flapping. By increasing the cost on leaf-to-spine links (e.g., using 'ip ospf cost' under the interface), the new spine's routes become less preferred, stabilizing the OSPF database and preventing transient loops. In real-world VXLAN EVPN fabrics, this is critical because underlay instability can cause overlay BGP sessions to flap, disrupting tenant traffic.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
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
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network — This question tests Network — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Increase the OSPF cost on the leaf-to-spine links. — When a new spine switch is added to a VXLAN EVPN fabric with an OSPF underlay, the leaf switches may experience routing instability because the new spine advertises routes with a lower cost, causing traffic to shift abruptly. Increasing the OSPF cost on the leaf-to-spine links makes those paths less preferred, stabilizing the routing table by preventing flapping and ensuring a more gradual convergence.
What should I do if I get this 350-601 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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