The answer is that lowering the spanning-tree path cost on one uplink of a nonroot switch makes that uplink more likely to be the root port. This is because STP path cost and root port selection are directly linked: a switch chooses the port with the lowest cumulative path cost to the root bridge as its root port, so reducing the cost on a specific uplink decreases the total cost to the root via that path, making it the most attractive candidate. On the CCNA 200-301 v2 exam, this concept tests your understanding that root port election is a local decision based on path cost, not a global election like the root bridge itself—a common trap is confusing this with bridge priority changes. Remember the mnemonic: "Lower cost on a nonroot link? That port will sink to the root."
CCNA Switching and Network Access Practice Question
This 200-301 practice question tests your understanding of switching and network access. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. A key principle to apply: spanning Tree Protocol (STP) selects the root port on a non-root switch as the port with the lowest path cost to the root bridge.. 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.
An engineer lowers the spanning-tree path cost on one uplink of a nonroot switch. What is the expected result if all else stays equal?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
That uplink becomes more likely to be the root port.
Lowering the path cost on a nonroot switch's uplink makes that link more attractive to the root bridge, increasing its likelihood of being selected as the root port. Option A is wrong because root bridge election depends on bridge priority and MAC address, not local path cost changes. Option C is false because designated ports on downstream switches are determined by their own topology and are unaffected by cost changes on an upstream nonroot switch's uplink. Option D is incorrect because BPDU transmission is governed by STP timers and port roles, not by adjusting path cost.
Key principle: Spanning Tree Protocol (STP) selects the root port on a non-root switch as the port with the lowest path cost to the root bridge.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The switch becomes the root bridge immediately.
Why it's wrong here
Path cost on one port does not by itself make the switch the root bridge.
When this WOULD be correct
In a different scenario where a question asks about the effects of changing bridge priority or bridge ID on a switch, lowering the spanning-tree path cost could indirectly lead to a switch becoming the root bridge if it results in a lower overall bridge ID compared to other switches.
✓
That uplink becomes more likely to be the root port.
Why this is correct
Correct. Lower root-path cost is preferred.
Related concept
Spanning Tree Protocol (STP) selects the root port on a non-root switch as the port with the lowest path cost to the root bridge.
✗
All designated ports on downstream switches immediately recalculate their port roles.
Why it's wrong here
Changing one port cost does not cause that blanket behavior.
When this WOULD be correct
In a different scenario where a switch is configured with multiple uplinks and a misconfiguration causes a loop, a question might ask what happens when a switch's designated port is incorrectly configured to block all traffic. In that case, all designated ports on downstream switches would indeed transition to a blocking state to prevent loops.
✗
The switch will stop transmitting BPDUs on that port until convergence is complete.
If the question were about a scenario where a port is administratively shut down or configured to not participate in spanning tree, then stating that the port stops sending BPDUs would be correct, as it would no longer engage in the spanning-tree protocol.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓That uplink becomes more likely to be the root port.Correct answer▾
Why this is correct
Correct. Lower root-path cost is preferred.
✗The switch becomes the root bridge immediately.Wrong answer — click to see why▾
Why this is wrong here
The root bridge is determined by the lowest bridge ID (priority + MAC address) among all switches. Changing the path cost on a single port does not affect the switch's bridge ID, so it cannot become the root bridge immediately.
★ When this WOULD be the correct answer
In a different scenario where a question asks about the effects of changing bridge priority or bridge ID on a switch, lowering the spanning-tree path cost could indirectly lead to a switch becoming the root bridge if it results in a lower overall bridge ID compared to other switches.
Why candidates choose this
Test-takers may confuse path cost with bridge priority, thinking that lowering cost makes the switch more likely to become root, but root election is based on bridge ID, not path cost.
✗All designated ports on downstream switches immediately recalculate their port roles.Wrong answer — click to see why▾
Why this is wrong here
A local cost change on a nonroot switch does not trigger an immediate role recalculation on all downstream switches; STP convergence is more limited.
★ When this WOULD be the correct answer
In a different scenario where a switch is configured with multiple uplinks and a misconfiguration causes a loop, a question might ask what happens when a switch's designated port is incorrectly configured to block all traffic. In that case, all designated ports on downstream switches would indeed transition to a blocking state to prevent loops.
Why candidates choose this
Students might overestimate the impact of a single cost change, thinking it triggers a widespread STP recalculation that forces many ports into blocking, similar to a topology change event.
✗The switch will stop transmitting BPDUs on that port until convergence is complete.Wrong answer — click to see why▾
Why this is wrong here
BPDU transmission continues regardless of cost adjustments; only port role changes or failures stop BPDUs.
★ When this WOULD be the correct answer
If the question were about a scenario where a port is administratively shut down or configured to not participate in spanning tree, then stating that the port stops sending BPDUs would be correct, as it would no longer engage in the spanning-tree protocol.
Why candidates choose this
Students might think that changing path cost affects BPDU transmission, confusing it with PortFast or BPDU guard features that disable BPDU handling on certain ports.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Remember, path cost adjustments influence root port selection, not root bridge election or port blocking.
Detailed technical explanation
How to think about this question
Spanning Tree Protocol (STP) is a Layer 2 protocol designed to prevent switching loops in Ethernet networks by creating a loop-free logical topology. STP elects a single root bridge based on the lowest bridge ID, and all other switches determine their best path to this root bridge. Each non-root switch selects one root port, which is the port with the lowest cumulative path cost to the root bridge. The path cost is calculated based on the bandwidth of the links along the path, with lower costs preferred.
When an engineer lowers the spanning-tree path cost on one uplink port of a non-root switch, this action reduces the cumulative cost to reach the root bridge via that port. As a result, the switch is more likely to select that uplink as its root port because STP always prefers the lowest-cost path to the root. This change affects only the local switch’s root port selection and does not influence the root bridge election, which depends solely on bridge IDs. Other ports on downstream switches remain unaffected in their designated or blocking states.
A common exam trap is to confuse path cost changes with root bridge election or assume that lowering path cost causes all designated ports downstream to block. In reality, path cost adjustments influence only root port selection on the local switch. Additionally, ports continue to send BPDUs to maintain STP topology information regardless of cost changes. Understanding this distinction helps avoid mistakes during the exam and in practical network troubleshooting, ensuring correct interpretation of STP behavior when modifying path costs.
KKey Concepts to Remember
Spanning Tree Protocol (STP) selects the root port on a non-root switch as the port with the lowest path cost to the root bridge.
Lowering the spanning-tree path cost on an uplink port makes that port more likely to be chosen as the root port by the switch.
The root bridge is elected based on the lowest bridge ID, not by adjusting path costs on individual switches.
Designated ports on downstream switches forward frames unless blocked by STP to prevent loops; changing one port cost does not block all designated ports.
Ports continue to send Bridge Protocol Data Units (BPDUs) to maintain STP topology information regardless of path cost changes.
STP path cost is cumulative and calculated based on interface bandwidth, influencing root port selection on non-root switches.
A non-root switch uses the lowest root path cost to determine its root port, which is the port that provides the best path to the root bridge.
Changing the path cost on one uplink affects only the local switch’s root port selection and does not immediately change the root bridge election.
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
Spanning Tree Protocol (STP) selects the root port on a non-root switch as the port with the lowest path cost to the root bridge.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Review spanning Tree Protocol (STP) selects the root port on a non-root switch as the port with the lowest path cost to the root bridge., then practise related 200-301 questions on the same topic to reinforce the concept.
Switching and Network Access — This question tests Switching and Network Access — Spanning Tree Protocol (STP) selects the root port on a non-root switch as the port with the lowest path cost to the root bridge..
What is the correct answer to this question?
The correct answer is: That uplink becomes more likely to be the root port. — Lowering the path cost on a nonroot switch's uplink makes that link more attractive to the root bridge, increasing its likelihood of being selected as the root port. Option A is wrong because root bridge election depends on bridge priority and MAC address, not local path cost changes. Option C is false because designated ports on downstream switches are determined by their own topology and are unaffected by cost changes on an upstream nonroot switch's uplink. Option D is incorrect because BPDU transmission is governed by STP timers and port roles, not by adjusting path cost.
What should I do if I get this 200-301 question wrong?
Review spanning Tree Protocol (STP) selects the root port on a non-root switch as the port with the lowest path cost to the root bridge., then practise related 200-301 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
Spanning Tree Protocol (STP) selects the root port on a non-root switch as the port with the lowest path cost to the root bridge.
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