The answer is that the port is not configured with BPDUguard. This is correct because Spanning Tree PortFast immediately transitions an access port to the forwarding state, bypassing the listening and learning phases to speed up connectivity. However, PortFast alone provides no loop prevention; if a rogue server or device sends BPDUs or bridges traffic between two switch ports, the switch will process those BPDUs and may re-enter STP convergence, creating a loop. On the Cisco DCCOR and CCNP Data Center Core 350-601 exam, this scenario tests your understanding that PortFast and BPDUguard are complementary features—PortFast accelerates port state changes, while BPDUguard protects against loops by err-disabling the port upon receiving any BPDU. A common trap is assuming PortFast alone prevents loops, but it only skips STP timers. Memory tip: think of PortFast as the “fast lane” and BPDUguard as the “guardrail”—without the guardrail, a crash (loop) is inevitable.
350-601 Network Practice Question
This 350-601 practice question tests your understanding of network. 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.
A data center switch has multiple access ports configured with spanning-tree portfast. A new server is connected to one of these ports and immediately causes a network loop. 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.
Clue: "immediately / without restart"
Why it matters: Time or reboot constraint — the correct answer must take effect right away without requiring a reboot or reload.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The port is not configured with BPDUguard.
Option C is correct because Spanning Tree Protocol (STP) PortFast immediately transitions a port to the forwarding state, bypassing the listening and learning phases. However, PortFast alone does not protect against loops if a rogue device (like a server) inadvertently connects two switch ports or sends BPDUs. BPDUguard must be explicitly configured on the port to disable it upon receiving any BPDU, preventing a loop. Without BPDUguard, the switch will process the BPDU and may re-enter STP convergence, potentially causing a loop if the server is misconfigured or bridging traffic.
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.
✗
The portfast feature is disabled by default on the switch.
Why it's wrong here
Portfast must be explicitly configured; it is not enabled by default.
✗
The server sends BPDUs.
Why it's wrong here
Portfast does not prevent BPDU reception; BPDUguard does.
✓
The port is not configured with BPDUguard.
Why this is correct
Without BPDUguard, the port stays up when a BPDU is received, potentially causing a loop.
Clue confirmation
The clue words "most likely", "immediately / without restart" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The switch is running MSTP instead of PVST+.
Why it's wrong here
MSTP itself does not cause loops; the problem is lack of BPDUguard.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that PortFast alone prevents loops, but the trap here is that PortFast only speeds up initial convergence; without BPDUguard, a PortFast port can still participate in STP and cause a loop if it receives a BPDU.
Detailed technical explanation
How to think about this question
Under the hood, PortFast relies on the assumption that no BPDUs will be received on an access port; if a BPDU is received, the port reverts to normal STP behavior, which can take up to 50 seconds (listening + learning) and may create a temporary loop if the server is bridging. BPDUguard, when configured, immediately err-disables the port upon BPDU reception, eliminating the loop risk. In real-world scenarios, a server with a software bridge (e.g., Hyper-V or VMware) or a misconfigured NIC team can inadvertently send BPDUs, making BPDUguard essential for all PortFast-enabled access ports.
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 practitioner preparing for the 350-601 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. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network — This question tests Network — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The port is not configured with BPDUguard. — Option C is correct because Spanning Tree Protocol (STP) PortFast immediately transitions a port to the forwarding state, bypassing the listening and learning phases. However, PortFast alone does not protect against loops if a rogue device (like a server) inadvertently connects two switch ports or sends BPDUs. BPDUguard must be explicitly configured on the port to disable it upon receiving any BPDU, preventing a loop. Without BPDUguard, the switch will process the BPDU and may re-enter STP convergence, potentially causing a loop if the server is misconfigured or bridging traffic.
What should I do if I get this 350-601 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely", "immediately / without restart". 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?
Read the scenario before looking for a memorised answer.
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Question Discussion
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