This 200-301 practice question tests your understanding of switching and network access. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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
SW1# show interfaces gigabitEthernet 0/1 status
Port Name Status Vlan Duplex Speed Type
Gi0/1 err-disabled 1 auto auto 10/100/1000BaseTX
SW1# show running-config interface gigabitEthernet 0/1
Building configuration...
Current configuration : 109 bytes
!
interface GigabitEthernet0/1
switchport mode access
spanning-tree bpduguard enable
end
SW1# show spanning-tree interface gigabitEthernet 0/1 detail
Port 1 (GigabitEthernet0/1) of VLAN0001 is broken (BPDU Guard)
Port path cost 4, Port priority 128, Port Identifier 128.1.
Designated root has priority 32768, address aaaa.bbbb.cccc
Designated bridge has priority 32768, address aaaa.bbbb.cccc
Designated port id is 128.1, designated path cost 0
Timers: message age 0, forward delay 0, hold 0
BPDU: sent 0, received 3
A network engineer notices that a new switch, SW3, was connected to port GigabitEthernet0/1 on SW1, but the port immediately went into an err-disabled state. The network uses Rapid PVST+ with BPDU Guard enabled globally on all access ports. The engineer checks the logs and sees 'bpduguard error detected' messages. What is the most likely cause of the err-disabled state?
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.
Exhibit
SW1# show interfaces gigabitEthernet 0/1 status
Port Name Status Vlan Duplex Speed Type
Gi0/1 err-disabled 1 auto auto 10/100/1000BaseTX
SW1# show running-config interface gigabitEthernet 0/1
Building configuration...
Current configuration : 109 bytes
!
interface GigabitEthernet0/1
switchport mode access
spanning-tree bpduguard enable
end
SW1# show spanning-tree interface gigabitEthernet 0/1 detail
Port 1 (GigabitEthernet0/1) of VLAN0001 is broken (BPDU Guard)
Port path cost 4, Port priority 128, Port Identifier 128.1.
Designated root has priority 32768, address aaaa.bbbb.cccc
Designated bridge has priority 32768, address aaaa.bbbb.cccc
Designated port id is 128.1, designated path cost 0
Timers: message age 0, forward delay 0, hold 0
BPDU: sent 0, received 3
A
The port is configured as an access port, but BPDU Guard should be disabled on all access ports.
Why wrong: BPDU Guard is typically enabled on access ports to prevent rogue switches; disabling it globally would reduce security.
B
A BPDU was received on port GigabitEthernet0/1, triggering BPDU Guard.
BPDU Guard err-disables a port immediately when a BPDU is received on an access port where it is enabled, which is exactly the scenario described.
C
Configure Root Guard on the interface to prevent the err-disabled state.
Why wrong: Root Guard prevents a port from becoming a root port, but it does not prevent err-disabled; it causes a root-inconsistent state.
D
Enable Loop Guard on the interface to prevent the err-disabled state.
Why wrong: Loop Guard prevents alternate or root ports from becoming designated when BPDUs are lost, but it does not prevent err-disabled from BPDU Guard.
The answer is that a BPDU was received on the port, triggering BPDU Guard and causing the err-disabled state. BPDU Guard is a security feature designed to protect the spanning-tree topology by immediately error-disabling any access port that receives a Bridge Protocol Data Unit, which should never occur on a port configured as an access port. When the new switch SW3 was connected, it likely sent BPDUs as part of Rapid PVST+ operation, and SW1’s globally enabled BPDU Guard on access ports detected those frames, shutting down GigabitEthernet0/1. On the CCNA 200-301 v2 exam, this scenario tests your understanding that BPDU Guard is intentionally aggressive—it err-disables, not just blocks—and a common trap is confusing it with Root Guard or Loop Guard, which serve different purposes. Remember the memory tip: BPDU Guard = “Guard the access port, shut it down fast.”
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
✓
A BPDU was received on port GigabitEthernet0/1, triggering BPDU Guard.
The err-disabled state is caused by BPDU Guard triggering when a BPDU is received on an access port. Option A is incorrect because BPDU Guard is intentionally enabled on access ports to prevent unauthorized switches from joining the network. Option C is wrong because Root Guard prevents a port from becoming the root, not from receiving BPDUs. Option D is wrong because Loop Guard prevents loops on blocked ports in case of unidirectional links, not relevant to BPDU reception.
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 port is configured as an access port, but BPDU Guard should be disabled on all access ports.
Why it's wrong here
BPDU Guard is typically enabled on access ports to prevent rogue switches; disabling it globally would reduce security.
✓
A BPDU was received on port GigabitEthernet0/1, triggering BPDU Guard.
Why this is correct
BPDU Guard err-disables a port immediately when a BPDU is received on an access port where it is enabled, which is exactly the scenario described.
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.
✗
Configure Root Guard on the interface to prevent the err-disabled state.
Why it's wrong here
Root Guard prevents a port from becoming a root port, but it does not prevent err-disabled; it causes a root-inconsistent state.
✗
Enable Loop Guard on the interface to prevent the err-disabled state.
Why it's wrong here
Loop Guard prevents alternate or root ports from becoming designated when BPDUs are lost, but it does not prevent err-disabled from BPDU Guard.
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.
✓A BPDU was received on port GigabitEthernet0/1, triggering BPDU Guard.Correct answer▾
Why this is correct
BPDU Guard err-disables a port immediately when a BPDU is received on an access port where it is enabled, which is exactly the scenario described.
✗The port is configured as an access port, but BPDU Guard should be disabled on all access ports.Wrong answer — click to see why▾
Why this is wrong here
BPDU Guard is correctly enabled on access ports; the issue is that an authorized switch was connected, so the guard was triggered unintentionally.
✗Configure Root Guard on the interface to prevent the err-disabled state.Wrong answer — click to see why▾
Why this is wrong here
Root Guard does not address the BPDU reception issue; it only affects root bridge selection.
✗Enable Loop Guard on the interface to prevent the err-disabled state.Wrong answer — click to see why▾
Why this is wrong here
Loop Guard would not have prevented the err-disabled; it is used for different scenarios.
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 confusion between BPDU Guard, Root Guard, and Loop Guard; candidates may incorrectly attribute the err-disable to Root Guard or Loop Guard, but the true cause is receiving a BPDU on a BPDU-Guard-enabled port.
Detailed technical explanation
How to think about this question
BPDU Guard is typically enabled globally with 'spanning-tree portfast bpduguard default', which applies to all PortFast-enabled ports. When a BPDU is received on a PortFast port, BPDU Guard err-disables the port to prevent loops. The err-disabled state can be recovered manually with 'shutdown' followed by 'no shutdown' or automatically with 'errdisable recovery cause bpduguard'. In real-world scenarios, connecting an authorized switch to an access port with BPDU Guard requires either disabling BPDU Guard on that port or configuring the port as a trunk.
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 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. 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.
Visual reference
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: A BPDU was received on port GigabitEthernet0/1, triggering BPDU Guard. — The err-disabled state is caused by BPDU Guard triggering when a BPDU is received on an access port. Option A is incorrect because BPDU Guard is intentionally enabled on access ports to prevent unauthorized switches from joining the network. Option C is wrong because Root Guard prevents a port from becoming the root, not from receiving BPDUs. Option D is wrong because Loop Guard prevents loops on blocked ports in case of unidirectional links, not relevant to BPDU reception.
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", "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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