CCNA Network Services and Security Practice Question
This 200-301 practice question tests your understanding of network services and security. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. A key principle to apply: bPDU Guard immediately disables a switch port if it receives a Bridge Protocol Data Unit (BPDU) on an edge port configured for host devices.. 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.
Exhibit
SW1# show errdisable recovery
ErrDisable Reason Timer Status
bpduguard Enabled
SW1# show interface status err-disabled
Port Name Status Reason
Gi1/0/11 err-disabled bpduguard
An access switch port shuts down as soon as a user connects a small unmanaged switch under the desk. Which feature caused that behavior?
SW1# show errdisable recovery
ErrDisable Reason Timer Status
bpduguard Enabled
SW1# show interface status err-disabled
Port Name Status Reason
Gi1/0/11 err-disabled bpduguard
A
Root Guard
Why wrong: Root Guard blocks a port from becoming root-inconsistent but does not typically err-disable an access edge port on BPDU reception.
B
BPDU Guard
BPDU Guard shuts an edge port down if it receives a BPDU.
C
Loop Guard
Why wrong: Loop Guard protects against missing BPDUs on non-designated ports.
D
Storm control
Why wrong: Storm control reacts to traffic thresholds, not BPDUs.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
BPDU Guard
BPDU Guard is meant to protect access ports by shutting them down if BPDUs are received. That usually means someone connected another switch where only an endpoint should exist.
Key principle: BPDU Guard immediately disables a switch port if it receives a Bridge Protocol Data Unit (BPDU) on an edge port configured for host devices.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Root Guard
Why it's wrong here
Root Guard blocks a port from becoming root-inconsistent but does not typically err-disable an access edge port on BPDU reception.
When this WOULD be correct
In a scenario where a question asks about preventing a switch port from becoming a root port due to a misconfigured switch or a rogue device, Root Guard would be the correct answer. For example, if a switch port is configured to only allow specific devices and a new switch attempts to become the root, Root Guard would shut down that port.
✓
BPDU Guard
Why this is correct
BPDU Guard shuts an edge port down if it receives a BPDU.
Related concept
BPDU Guard immediately disables a switch port if it receives a Bridge Protocol Data Unit (BPDU) on an edge port configured for host devices.
✗
Loop Guard
Why it's wrong here
Loop Guard protects against missing BPDUs on non-designated ports.
When this WOULD be correct
In a different question setup, if the scenario involved a network where a switch was misconfigured and created a loop, and the question asked about preventing loops, Loop Guard would be the correct answer as it would block the port to prevent broadcast storms.
✗
Storm control
Why it's wrong here
Storm control reacts to traffic thresholds, not BPDUs.
When this WOULD be correct
If the question were about a switch port experiencing high levels of broadcast traffic due to a misconfigured device, and the port was configured with storm control, then storm control would be the correct answer as it would limit the broadcast traffic and prevent network disruption.
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 GuardCorrect answer▾
Why this is correct
BPDU Guard shuts an edge port down if it receives a BPDU.
✗Root GuardWrong answer — click to see why▾
Why this is wrong here
Root Guard does not shut down a port upon receiving BPDUs; it only prevents the port from becoming a root port by placing it in a root-inconsistent state if a superior BPDU is received.
★ When this WOULD be the correct answer
In a scenario where a question asks about preventing a switch port from becoming a root port due to a misconfigured switch or a rogue device, Root Guard would be the correct answer. For example, if a switch port is configured to only allow specific devices and a new switch attempts to become the root, Root Guard would shut down that port.
Why candidates choose this
Students may confuse Root Guard with BPDU Guard because both involve BPDU handling, but Root Guard is designed to protect the spanning-tree root, not to prevent unauthorized switches.
✗Loop GuardWrong answer — click to see why▾
Why this is wrong here
Loop Guard is designed to detect and prevent loops caused by missing BPDUs on non-designated ports, not to shut down ports that receive BPDUs. It does not err-disable the port but instead places it in a loop-inconsistent state.
★ When this WOULD be the correct answer
In a different question setup, if the scenario involved a network where a switch was misconfigured and created a loop, and the question asked about preventing loops, Loop Guard would be the correct answer as it would block the port to prevent broadcast storms.
Why candidates choose this
The name 'Loop Guard' might suggest it prevents loops from unauthorized switches, but its actual function is to guard against unidirectional link failures that cause BPDU loss.
✗Storm controlWrong answer — click to see why▾
Why this is wrong here
Storm control monitors traffic levels (broadcast, multicast, or unicast) and drops or shuts down a port when a threshold is exceeded, but it does not react to BPDU reception.
★ When this WOULD be the correct answer
If the question were about a switch port experiencing high levels of broadcast traffic due to a misconfigured device, and the port was configured with storm control, then storm control would be the correct answer as it would limit the broadcast traffic and prevent network disruption.
Why candidates choose this
Since storm control can shut down a port, a student might think it could be triggered by BPDUs, but BPDUs are control frames and not subject to storm control thresholds.
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
A frequent exam trap is mistaking BPDU Guard for Root Guard or Loop Guard. Candidates often confuse Root Guard’s role in blocking superior BPDUs with BPDU Guard’s immediate port shutdown on any BPDU reception. Similarly, Loop Guard protects against missing BPDUs but does not err-disable ports. This confusion leads to selecting incorrect answers that do not cause the port to shut down when a small unmanaged switch is connected. Remember, only BPDU Guard err-disables the port upon receiving BPDUs on an access port, which is the exact behavior described in the question.
Detailed technical explanation
How to think about this question
BPDU Guard is a Cisco feature designed to protect the Spanning Tree Protocol (STP) topology by disabling ports that receive BPDUs unexpectedly. It is typically enabled on access ports that connect to end devices, which should never send BPDUs. When a BPDU is received on such a port, BPDU Guard immediately places the port into an err-disabled state, effectively shutting it down to prevent potential network loops or topology changes caused by unauthorized switches.
The decision process behind BPDU Guard is straightforward: if a port configured as an edge port (portfast enabled) receives a BPDU, it is considered a sign that a switch or bridge is connected where only a host should be. This triggers the err-disable action to protect the network. Other STP features like Root Guard and Loop Guard serve different purposes; Root Guard blocks ports from becoming root ports but does not shut them down, while Loop Guard prevents loops caused by missing BPDUs but also does not err-disable ports.
A common exam trap is confusing BPDU Guard with Root Guard or Loop Guard. Root Guard blocks superior BPDUs to maintain root bridge stability but does not disable ports. Loop Guard prevents loops from unidirectional link failures but does not shut down ports on BPDU reception. Storm Control is unrelated to BPDUs and focuses on traffic thresholds. Understanding that BPDU Guard specifically protects edge ports by disabling them upon BPDU receipt is critical for correctly answering questions about port shutdowns caused by connecting unmanaged switches.
KKey Concepts to Remember
BPDU Guard immediately disables a switch port if it receives a Bridge Protocol Data Unit (BPDU) on an edge port configured for host devices.
Root Guard prevents a port from becoming a root port by blocking superior BPDUs but does not err-disable the port upon BPDU reception.
Loop Guard protects against unidirectional link failures by preventing a port from transitioning to the forwarding state when BPDUs are missing.
Storm Control monitors traffic levels on a port and blocks traffic if thresholds are exceeded, but it does not react to BPDU frames.
Access ports are typically configured with BPDU Guard to protect the network from unauthorized switches that could cause spanning-tree topology changes.
BPDU Guard helps maintain network stability by err-disabling ports that receive unexpected BPDUs, indicating a potential switch connection where only end devices should be.
Spanning Tree Protocol (STP) uses BPDUs to detect loops and elect root bridges, and BPDU Guard enforces strict edge port behavior by shutting down ports receiving BPDUs.
Misconfiguration or unauthorized connection of unmanaged switches to access ports can trigger BPDU Guard, causing the port to shut down to prevent topology changes.
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
BPDU Guard immediately disables a switch port if it receives a Bridge Protocol Data Unit (BPDU) on an edge port configured for host devices.
Real-world example
How this comes up in practice
A practitioner preparing for the 200-301 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. BPDU Guard immediately disables a switch port if it receives a Bridge Protocol Data Unit (BPDU) on an edge port configured for host devices. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
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 bPDU Guard immediately disables a switch port if it receives a Bridge Protocol Data Unit (BPDU) on an edge port configured for host devices., then practise related 200-301 questions on the same topic to reinforce the concept.
Network Services and Security — This question tests Network Services and Security — BPDU Guard immediately disables a switch port if it receives a Bridge Protocol Data Unit (BPDU) on an edge port configured for host devices..
What is the correct answer to this question?
The correct answer is: BPDU Guard — BPDU Guard is meant to protect access ports by shutting them down if BPDUs are received. That usually means someone connected another switch where only an endpoint should exist.
What should I do if I get this 200-301 question wrong?
Review bPDU Guard immediately disables a switch port if it receives a Bridge Protocol Data Unit (BPDU) on an edge port configured for host devices., then practise related 200-301 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
BPDU Guard immediately disables a switch port if it receives a Bridge Protocol Data Unit (BPDU) on an edge port configured for host devices.
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