CCNA Switching and Network Access Practice Question
Exhibit: A user reports intermittent connectivity after a new switch was connected to an access port. Which feature would have prevented this by immediately disabling the port when a BPDU was received?
⚠ Common exam trap
Be cautious not to confuse BPDU Guard with other guard features like Root Guard or Loop Guard, which serve different purposes.
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 Guard
BPDU Guard is the correct answer because it protects PortFast-enabled edge ports by immediately disabling the port upon receiving a BPDU, preventing accidental loops. Root Guard prevents the port from becoming a root port, not from BPDU reception. Loop Guard prevents alternate or root ports from becoming designated due to BPDU loss, unrelated to BPDU reception disabling. UDLD detects unidirectional links but does not disable ports upon BPDU reception.
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 is not the right solution because it does not err-disable an access port on BPDU reception; instead, it places the port in a root-inconsistent state if a superior BPDU is received, blocking the port only while such BPDUs persist. In this scenario, the new device likely causes the port to shut down immediately via err-disable, which is inconsistent with Root Guard's behavior of simply blocking and then recovering when the BPDU stops.
When this WOULD be correct
In a different question, if the scenario involved preventing a non-root switch from becoming the root bridge while allowing the existing root bridge to maintain its status, Root Guard would be the correct answer. For example, if a user was asked how to secure the root bridge in a network with multiple switches, Root Guard would be appropriate.
- ✗
Loop Guard
Why it's wrong here
Loop Guard is not appropriate here because it addresses unidirectional link failures by monitoring the continued receipt of BPDUs on non-designated ports. It does not take action upon BPDU reception; rather, it blocks a port when BPDUs stop arriving. The reported intermittent connectivity stems from an unexpected BPDU arriving, not from a unidirectional link, so Loop Guard would neither detect nor resolve this issue.
When this WOULD be correct
In a different question, if the scenario involved a network topology where a switch was incorrectly configured and caused a loop, asking about preventing loops without specifying immediate port disablement could make Loop Guard the correct answer, as it would maintain stability by preventing loops.
- ✓
BPDU Guard
Why this is correct
BPDU Guard is the correct protection mechanism because it is designed specifically for PortFast-enabled access ports. When a legitimate BPDU is received on such a port, BPDU Guard immediately err-disables the interface to prevent a potential bridging loop, which explains the intermittent connectivity: the new device is sending BPDUs and triggering the shutdown each time.
- ✗
UDLD
Why it's wrong here
UDLD is irrelevant to this fault because it focuses on verifying bidirectional link connectivity on point-to-point fiber or wire links by exchanging UDLD probes. It does not inspect or react to BPDUs from an unexpected switch; therefore, a new device sending BPDUs would go unnoticed by UDLD, and the port would not be err-disabled, leaving the loop risk unresolved.
When this WOULD be correct
In a scenario where a question asks about how to prevent unidirectional links from causing network issues, and it specifies that a port should be disabled when a unidirectional link is detected, UDLD would be the correct answer.
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 is the correct protection mechanism because it is designed specifically for PortFast-enabled access ports. When a legitimate BPDU is received on such a port, BPDU Guard immediately err-disables the interface to prevent a potential bridging loop, which explains the intermittent connectivity: the new device is sending BPDUs and triggering the shutdown each time.
✗Root GuardWrong answer — click to see why▾
Why this is wrong here
Root Guard is designed to prevent a non-root switch from becoming the root bridge in a spanning tree topology by blocking ports that receive BPDUs from non-root devices. However, it does not disable the port immediately upon receiving a BPDU, which is the requirement in this scenario.
★ When this WOULD be the correct answer
In a different question, if the scenario involved preventing a non-root switch from becoming the root bridge while allowing the existing root bridge to maintain its status, Root Guard would be the correct answer. For example, if a user was asked how to secure the root bridge in a network with multiple switches, Root Guard would be appropriate.
Why candidates choose this
Candidates may confuse Root Guard with BPDU Guard due to their similar functions related to BPDUs, leading them to mistakenly believe that Root Guard also provides immediate port shutdown capabilities.
✗Loop GuardWrong answer — click to see why▾
Why this is wrong here
Loop Guard is designed to prevent loops in a network by keeping a port in a loop-inconsistent state if it stops receiving BPDUs. However, it does not disable the port upon receiving a BPDU, which is the requirement in this scenario.
★ When this WOULD be the correct answer
In a different question, if the scenario involved a network topology where a switch was incorrectly configured and caused a loop, asking about preventing loops without specifying immediate port disablement could make Loop Guard the correct answer, as it would maintain stability by preventing loops.
Why candidates choose this
Candidates may choose Loop Guard because they associate it with BPDU handling and network stability, leading them to believe it could address issues related to connectivity and loops, despite it not directly disabling ports upon BPDU reception.
✗UDLDWrong answer — click to see why▾
Why this is wrong here
UDLD (Unidirectional Link Detection) is designed to detect unidirectional links and disable the port if it identifies one, but it does not specifically address the issue of BPDUs being received on an access port, which is the focus of the question.
★ When this WOULD be the correct answer
In a scenario where a question asks about how to prevent unidirectional links from causing network issues, and it specifies that a port should be disabled when a unidirectional link is detected, UDLD would be the correct answer.
Why candidates choose this
Candidates may confuse UDLD with BPDU Guard due to both being related to link stability and port management, leading them to mistakenly believe UDLD could also prevent issues caused by BPDUs.
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?”
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Related to this question
Learn chapter
Configuring Switch Ports for Desktops, VoIP Phones, APs, IoT, and Virtualized Hosts
Key term
BPDU
A Bridge Protocol Data Unit is a layer 2 frame that switches use to exchange information about the Spanning Tree Protocol (STP) to prevent network loops.
Key term
Root Guard
Root Guard is a Spanning Tree Protocol (STP) feature that protects the root bridge placement on a network port to prevent unauthorized switches from becoming the root bridge.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.