The answer is BPDU Guard detected a BPDU on a PortFast-enabled port and disabled it. This is correct because BPDU Guard is a safety mechanism that immediately places a PortFast-configured access port into an err-disabled state upon receiving any BPDU, preventing an unauthorized switch from being connected and disrupting the spanning-tree topology. On the CCNA 200-301 v2 exam, this scenario tests your understanding of how Rapid PVST+ interacts with PortFast and BPDU Guard, and it is a common trap where candidates mistakenly blame root bridge elections or trunk misconfigurations. Remember that PortFast assumes no BPDUs should arrive on an access port, so BPDU Guard acts as a bouncer—if a BPDU shows up, the port gets kicked out immediately. A useful memory tip is “BPDU Guard = Bouncer: one BPDU, you’re done.”
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.
Exhibit
SwitchC# show interfaces status
Port Name Status Vlan Duplex Speed Type
Gi0/1 err-disabled 10 auto auto 10/100/1000BaseTX
Gi0/2 connected 10 a-full a-100 10/100/1000BaseTX
Gi0/3 connected 1 a-full a-100 10/100/1000BaseTX
SwitchC# show interfaces gigabitEthernet 0/1
GigabitEthernet0/1 is down, line protocol is down (err-disabled)
Hardware is Gigabit Ethernet, address is aaaa.bbbb.cccc (bia aaaa.bbbb.cccc)
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Auto-duplex, Auto-speed, media type is 10/100/1000BaseTX
input flow-control is off, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:00, output 00:00:00, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 1 interface resets
0 unknown protocol drops
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 pause output
0 output buffer failures, 0 output buffers swapped out
SwitchC# show errdisable recovery
ErrDisable Reason Timer Status
----------------- --------------
arp-inspection Disabled
bpduguard Enabled
channel-misconfig Disabled
...
SwitchC# show spanning-tree vlan 10
VLAN0010
Spanning tree enabled protocol rstp
Root ID Priority 32778
Address aaaa.bbbb.cccc
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address aaaa.bbbb.cccc
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/2 Desg FWD 4 128.2 P2p
Gi0/3 Desg FWD 4 128.3 P2p
SwitchC# show running-config | section interface GigabitEthernet0/1
interface GigabitEthernet0/1
switchport mode access
switchport access vlan 10
spanning-tree portfast
spanning-tree bpduguard enable
A network administrator is troubleshooting connectivity loss in a switched network. All switches run Rapid PVST+. A host connected to an access port on SwitchC can no longer reach the default gateway. The access port is configured with PortFast and BPDU Guard. The administrator checks the interface status and finds it in an err-disabled state. What is the most likely cause of this issue?
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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
BPDU Guard detected a BPDU on a PortFast-enabled port and disabled it.
B is correct because BPDU Guard is designed to protect the spanning-tree topology by disabling a PortFast-enabled port if it receives a BPDU, placing the port in err-disabled state. Option A is incorrect: a root bridge election failure would not cause a port to err-disable; loops do not directly trigger this state without BPDU Guard. Option C is incorrect because PortFast and BPDU Guard work with all spanning-tree variants including Rapid PVST+. Option D is incorrect: a trunk misconfiguration alone would not cause err-disable unless BPDU Guard detects a BPDU on a PortFast port.
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 root bridge election failed, causing a loop.
Why it's wrong here
The root bridge is correctly elected (SwitchC is root for VLAN 10).
✓
BPDU Guard detected a BPDU on a PortFast-enabled port and disabled it.
Why this is correct
BPDU Guard is enabled on Gi0/1, and a BPDU was received, causing the port to go err-disabled.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Rapid PVST+ is not compatible with PortFast.
Why it's wrong here
Rapid PVST+ and PortFast are compatible; PortFast is designed for access ports in Rapid PVST+.
✗
The port is configured as a trunk but should be an access port.
Why it's wrong here
The port is configured as an access port (switchport mode access), not a trunk.
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 Guard detected a BPDU on a PortFast-enabled port and disabled it.Correct answer▾
Why this is correct
BPDU Guard is enabled on Gi0/1, and a BPDU was received, causing the port to go err-disabled.
✗The root bridge election failed, causing a loop.Wrong answer — click to see why▾
Why this is wrong here
The root bridge election did not fail; SwitchC is correctly elected as root for VLAN 10. A loop would not cause an err-disabled state; loops typically cause high CPU or port flapping, not err-disable.
Why candidates choose this
Students may associate connectivity loss with root bridge issues or loops, but the err-disabled state specifically points to a port security feature like BPDU Guard.
✗Rapid PVST+ is not compatible with PortFast.Wrong answer — click to see why▾
Why this is wrong here
Rapid PVST+ and PortFast are fully compatible; PortFast is specifically designed to work with Rapid PVST+ to allow access ports to transition immediately to forwarding state. There is no incompatibility.
Why candidates choose this
Some might think that Rapid PVST+ requires BPDUs on all ports, but PortFast is a standard feature that bypasses the listening/learning states and works with any spanning-tree mode.
✗The port is configured as a trunk but should be an access port.Wrong answer — click to see why▾
Why this is wrong here
The port is configured as an access port (switchport mode access), not a trunk. A trunk misconfiguration would not cause an err-disabled state due to BPDU Guard; it might cause a different issue like VLAN mismatch.
Why candidates choose this
Students might confuse access port configuration with trunk issues, especially if they think the default gateway is on a different VLAN. However, the err-disabled state is directly caused by BPDU Guard.
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
Cisco often tests the misconception that PortFast and BPDU Guard are incompatible with Rapid PVST+, but in reality, PortFast is a port-level feature that works identically across all spanning-tree variants, and BPDU Guard is the mechanism that causes the err-disabled state when a BPDU is received.
Detailed technical explanation
How to think about this question
BPDU Guard works by monitoring PortFast-enabled ports for incoming BPDUs; if a BPDU is received, the port is immediately placed into an err-disabled state to prevent a potential bridging loop. This is commonly used on access ports where end devices like PCs or printers are connected, and if a switch is mistakenly plugged into such a port, BPDU Guard protects the network. In a real-world scenario, an attacker could also send forged BPDUs to cause a denial-of-service by triggering BPDU Guard on multiple ports, which is why it is often paired with BPDU Filtering for additional control.
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 help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Switching and Network Access — This question tests Switching and Network Access — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: BPDU Guard detected a BPDU on a PortFast-enabled port and disabled it. — B is correct because BPDU Guard is designed to protect the spanning-tree topology by disabling a PortFast-enabled port if it receives a BPDU, placing the port in err-disabled state. Option A is incorrect: a root bridge election failure would not cause a port to err-disable; loops do not directly trigger this state without BPDU Guard. Option C is incorrect because PortFast and BPDU Guard work with all spanning-tree variants including Rapid PVST+. Option D is incorrect: a trunk misconfiguration alone would not cause err-disable unless BPDU Guard detects a BPDU on a PortFast port.
What should I do if I get this 200-301 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". 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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. You are connected to SW1, a Layer 2 switch. The network administrator wants to prevent unauthorized switches from being connected to access ports. Port G0/1 is an access port in VLAN 10. You need to configure BPDU Guard on this port to protect against STP loops caused by rogue switches. Additionally, enable PortFast for immediate transition to forwarding.
Why A: PortFast allows an access port to skip STP listening/learning and transition immediately to forwarding. BPDU Guard protects against STP loops by error-disabling the port if a BPDU is received, which would indicate an unauthorized switch connection.
Last reviewed: Jun 11, 2026
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