- A
Spanning tree is disabled globally, allowing the rogue switch to create a loop.
Why wrong: If spanning tree were disabled, BPDU guard would also not function, but this is a less likely cause because the scenario shows BPDU guard is enabled, which implies STP is running. Additionally, a technician would likely notice if STP was disabled.
- B
BPDU guard is misconfigured on the wrong ports, so it failed to block the rogue switch.
Why wrong: BPDU guard, when enabled globally, applies automatically to all PortFast-enabled ports. The misconfiguration would mean it was not applied, but the technician confirmed it is globally enabled, so this is not the cause.
- C
Root guard is incorrectly enabled on the access ports, causing the rogue switch to become the root bridge.
Why wrong: Root guard prevents a port from becoming a root port by ignoring superior BPDUs. If enabled, it would not cause a broadcast storm; it would simply block the port if a better BPDU was received. It does not prevent loop detection by STP.
- D
BPDU filter is globally enabled, causing the switch to suppress BPDUs on PortFast ports and preventing BPDU guard from triggering.
Global BPDU filter on a switch sets PortFast on all access ports and disables BPDU transmission and reception on those ports. The rogue switch’s BPDUs are never processed, so BPDU guard—which depends on receiving a BPDU—never err-disables the port, allowing a loop and broadcast storm.
Quick Answer
The answer is that BPDU filter globally enabled prevents BPDU Guard from detecting loops, causing a broadcast storm. This occurs because when BPDU filter is enabled globally, it automatically enables PortFast on all access ports and suppresses both the sending and receiving of BPDUs on those ports. As a result, even though BPDU Guard is globally enabled, it never sees a BPDU from a rogue switch to trigger the err-disable state, allowing a bridging loop to form and flood the network with broadcast traffic. On the CCNA 200-301 v2 exam, this scenario tests your understanding of the critical interaction between PortFast, BPDU Guard, and BPDU filter—a common trap is assuming global BPDU Guard alone prevents loops, when in fact global BPDU filter overrides it by silencing all BPDU activity. A helpful memory tip: “Filter first, Guard last—if BPDUs can’t pass, Guard won’t blast.”
CCNA Switching and Network Access Practice Question
This 200-301 practice question tests your understanding of switching and network access. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
A technician is troubleshooting a network-wide broadcast storm that has caused severe performance issues. The technician notices that BPDU guard is globally enabled on the access layer switch, but no ports are in an err-disabled state. All access ports have PortFast enabled. 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.
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
BPDU filter is globally enabled, causing the switch to suppress BPDUs on PortFast ports and preventing BPDU guard from triggering.
When BPDU filter is enabled globally on a switch, it enables PortFast on all access ports and also prevents those ports from sending or receiving BPDUs. If a rogue switch is then connected to such a port, the switch does not detect any BPDU from it, so BPDU guard never triggers despite being enabled globally. This allows the rogue switch to create a bridging loop without STP intervention, leading to a broadcast storm.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Spanning tree is disabled globally, allowing the rogue switch to create a loop.
Why it's wrong here
If spanning tree were disabled, BPDU guard would also not function, but this is a less likely cause because the scenario shows BPDU guard is enabled, which implies STP is running. Additionally, a technician would likely notice if STP was disabled.
- ✗
BPDU guard is misconfigured on the wrong ports, so it failed to block the rogue switch.
Why it's wrong here
BPDU guard, when enabled globally, applies automatically to all PortFast-enabled ports. The misconfiguration would mean it was not applied, but the technician confirmed it is globally enabled, so this is not the cause.
- ✗
Root guard is incorrectly enabled on the access ports, causing the rogue switch to become the root bridge.
Why it's wrong here
Root guard prevents a port from becoming a root port by ignoring superior BPDUs. If enabled, it would not cause a broadcast storm; it would simply block the port if a better BPDU was received. It does not prevent loop detection by STP.
- ✓
BPDU filter is globally enabled, causing the switch to suppress BPDUs on PortFast ports and preventing BPDU guard from triggering.
Why this is correct
Global BPDU filter on a switch sets PortFast on all access ports and disables BPDU transmission and reception on those ports. The rogue switch’s BPDUs are never processed, so BPDU guard—which depends on receiving a BPDU—never err-disables the port, allowing a loop and broadcast storm.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
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.
✓BPDU filter is globally enabled, causing the switch to suppress BPDUs on PortFast ports and preventing BPDU guard from triggering.Correct answer▾
Why this is correct
Global BPDU filter on a switch sets PortFast on all access ports and disables BPDU transmission and reception on those ports. The rogue switch’s BPDUs are never processed, so BPDU guard—which depends on receiving a BPDU—never err-disables the port, allowing a loop and broadcast storm.
✗Spanning tree is disabled globally, allowing the rogue switch to create a loop.Wrong answer — click to see why▾
Why this is wrong here
This answer assumes STP is off entirely, but the presence of BPDU guard configuration indicates spanning tree is operational.
✗BPDU guard is misconfigured on the wrong ports, so it failed to block the rogue switch.Wrong answer — click to see why▾
Why this is wrong here
Candidates often assume that BPDU guard simply failed, overlooking the interaction with BPDU filter, which can neutralize guard by suppressing BPDUs.
✗Root guard is incorrectly enabled on the access ports, causing the rogue switch to become the root bridge.Wrong answer — click to see why▾
Why this is wrong here
Root guard is a different feature and not related to the suppression of BPDUs that would allow a loop to form undetected.
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: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Trap categories for this question
Command / output trap
If spanning tree were disabled, BPDU guard would also not function, but this is a less likely cause because the scenario shows BPDU guard is enabled, which implies STP is running. Additionally, a technician would likely notice if STP was disabled.
Scenario analysis trap
If spanning tree were disabled, BPDU guard would also not function, but this is a less likely cause because the scenario shows BPDU guard is enabled, which implies STP is running. Additionally, a technician would likely notice if STP was disabled.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
- CIDR notation defines the prefix length.
- Block size helps identify subnet boundaries.
- Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
- The required host count determines the smallest suitable subnet.
TExam Day Tips
- Write the block size before choosing the subnet.
- Check whether the question asks for hosts, subnets or a specific address range.
- Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 200-301 subnetting questions on CIDR, address ranges, and subnet selection.
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Switching and Network Access — study guide chapter
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FAQ
Questions learners often ask
What does this 200-301 question test?
Switching and Network Access — This question tests Switching and Network Access — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: BPDU filter is globally enabled, causing the switch to suppress BPDUs on PortFast ports and preventing BPDU guard from triggering. — When BPDU filter is enabled globally on a switch, it enables PortFast on all access ports and also prevents those ports from sending or receiving BPDUs. If a rogue switch is then connected to such a port, the switch does not detect any BPDU from it, so BPDU guard never triggers despite being enabled globally. This allows the rogue switch to create a bridging loop without STP intervention, leading to a broadcast storm.
What should I do if I get this 200-301 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 200-301 subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Last reviewed: Jun 14, 2026
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.
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