- A
Disable PortFast on all access ports to force proper RSTP convergence.
Why wrong: Incorrect: PortFast does not cause flapping; it only affects edge ports.
- B
Configure spanning-tree vlan 10 priority 4096 on DS1 to ensure it remains the root and that AS2 selects the uplink to DS1 as root port.
Correct: Lowering priority ensures DS1 is root and AS2's link to DS1 becomes the root port, stopping flapping.
- C
Increase DS2's priority to 8192 to make it less likely to become the root.
Why wrong: Incorrect: DS1 is already root; priority change on DS2 won't affect AS2's root port selection if both paths have equal cost.
- D
Change the fiber link between AS2 and DS1 to operate at 10 Gbps to reduce its cost.
Why wrong: Incorrect: Changing speed may not be feasible and doesn't address the root cause of equal cost.
Quick Answer
The answer is to configure `spanning-tree vlan 10 priority 4096` on DS1. This resolves the Rapid PVST+ root port flapping because both uplinks from AS2 to DS1 and DS2 have equal cost (4 for 1 Gbps), causing the switch to continuously re-evaluate which port offers the superior root path. By lowering DS1’s bridge priority below the default 32768, you guarantee DS1 remains the root bridge for VLAN 10, making the direct uplink to DS1 the single best path and stabilizing AS2’s root port selection. On the ENCOR 350-401 exam, this scenario tests your understanding of Rapid PVST+ path selection and the impact of equal-cost links on root port stability—a common trap is assuming PortFast or UplinkFast alone fixes flapping, but the root cause here is identical default costs. Remember: when root ports flap due to equal cost, the fix is not to change the link cost but to adjust the root bridge priority to break the tie. Memory tip: “Priority 4096 keeps the root fixed, so the flapping gets nixed.”
CCNP Infrastructure Practice Question
This 350-401 practice question tests your understanding of infrastructure. 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.
Your company has a campus network with two distribution switches (DS1 and DS2) each connected to two access switches (AS1-AS4). All switches run Rapid PVST+. The root bridge for VLAN 10 is DS1. Recently, users on AS2 (VLAN 10) report intermittent connectivity. You notice that AS2's root port for VLAN 10 is flapping between two uplinks to DS1 and DS2. The link from AS2 to DS1 is a 1 Gbps fiber, and the link to DS2 is a 1 Gbps copper. The cost of both links is 4 (default for 1 Gbps). The network administrator previously configured PortFast on all access ports but did not configure any other spanning-tree parameters. Which action should you take to stabilize the topology?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Configure spanning-tree vlan 10 priority 4096 on DS1 to ensure it remains the root and that AS2 selects the uplink to DS1 as root port.
The root port flapping occurs because both uplinks from AS2 to DS1 and DS2 have identical default costs (4 for 1 Gbps), causing Rapid PVST+ to continuously re-evaluate which port is the superior root port. By configuring `spanning-tree vlan 10 priority 4096` on DS1, you lower its bridge priority below the default 32768, ensuring DS1 remains the root bridge for VLAN 10. This makes the path through DS1 the single best root path, stabilizing AS2's root port selection.
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.
- ✗
Disable PortFast on all access ports to force proper RSTP convergence.
Why it's wrong here
Incorrect: PortFast does not cause flapping; it only affects edge ports.
- ✓
Configure spanning-tree vlan 10 priority 4096 on DS1 to ensure it remains the root and that AS2 selects the uplink to DS1 as root port.
Why this is correct
Correct: Lowering priority ensures DS1 is root and AS2's link to DS1 becomes the root port, stopping flapping.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Increase DS2's priority to 8192 to make it less likely to become the root.
Why it's wrong here
Incorrect: DS1 is already root; priority change on DS2 won't affect AS2's root port selection if both paths have equal cost.
- ✗
Change the fiber link between AS2 and DS1 to operate at 10 Gbps to reduce its cost.
Why it's wrong here
Incorrect: Changing speed may not be feasible and doesn't address the root cause of equal cost.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that changing the root bridge priority on the current root is unnecessary because it is already the root, but the trap here is that equal-cost paths cause flapping, and the correct fix is to ensure the root bridge has a lower priority so that the path cost calculation is deterministic.
Detailed technical explanation
How to think about this question
In Rapid PVST+, the root port selection process uses the lowest root path cost to the root bridge; when two ports have equal cost, the bridge ID (priority + MAC) of the neighbor is used as a tiebreaker. Since both DS1 and DS2 have the same default priority (32768), the MAC address becomes the deciding factor, which can cause flapping if the MAC addresses are close or if the topology changes slightly. Configuring a lower priority on DS1 (e.g., 4096) ensures it is the root and that the path through DS1 has a lower cumulative cost, eliminating the tie and stabilizing the root port.
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 help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
What to study next
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FAQ
Questions learners often ask
What does this 350-401 question test?
Infrastructure — This question tests Infrastructure — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Configure spanning-tree vlan 10 priority 4096 on DS1 to ensure it remains the root and that AS2 selects the uplink to DS1 as root port. — The root port flapping occurs because both uplinks from AS2 to DS1 and DS2 have identical default costs (4 for 1 Gbps), causing Rapid PVST+ to continuously re-evaluate which port is the superior root port. By configuring `spanning-tree vlan 10 priority 4096` on DS1, you lower its bridge priority below the default 32768, ensuring DS1 remains the root bridge for VLAN 10. This makes the path through DS1 the single best root path, stabilizing AS2's root port selection.
What should I do if I get this 350-401 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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Last reviewed: Jun 11, 2026
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