The answer is that root guard is enabled and the port received a superior BPDU, causing it to become root-inconsistent. This occurs because root guard, when configured on a switch port, actively monitors incoming BPDUs; if it detects a superior BPDU—one that would force the local switch to become a non-root bridge—it immediately places the port into a root-inconsistent blocking state to protect the current root bridge election. In the CCNA 200-301 v2 exam, this scenario tests your understanding of how STP security features like root guard differ from BPDU guard: root guard reacts to superior BPDUs, not to unexpected BPDUs, and it blocks the port while keeping it administratively up. A common trap is confusing this state with a port down or err-disabled condition, but the interface will show as up/up yet blocked by spanning tree. Remember the mnemonic: “Root guard rejects a superior root—blocking the port to protect the throne.”
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
SwitchA# show spanning-tree interface gigabitEthernet 0/1 detail
Port 1 (GigabitEthernet0/1) of VLAN0010 is root INCONSISTENT
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
Number of transitions to forwarding state: 0
The port is not in the portfast mode
Root guard is enabled on the port
BPDU guard is disabled
Loop guard is disabled
Link type is point-to-point (auto)
BPDU: sent 3, received 2
A network engineer receives a call that users in VLAN 10 on Switch B cannot ping the default gateway, which is a router on a stick connected to Switch A. The engineer checks the Spanning Tree Protocol state on the interface connecting Switch A to Switch B (GigabitEthernet0/1) and finds it is in a root-inconsistent state. Which command output best explains the cause of the issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Clue: "which command"
Why it matters: Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
SwitchA# show spanning-tree interface gigabitEthernet 0/1 detail
Port 1 (GigabitEthernet0/1) of VLAN0010 is root INCONSISTENT
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
Number of transitions to forwarding state: 0
The port is not in the portfast mode
Root guard is enabled on the port
BPDU guard is disabled
Loop guard is disabled
Link type is point-to-point (auto)
BPDU: sent 3, received 2
A
The interface is in err-disable state due to BPDU guard.
Why wrong: BPDU guard would place the port in err-disable if a BPDU is received on a PortFast-enabled port.
B
Root guard is enabled and the port received a superior BPDU, causing it to become root-inconsistent.
Root guard on the interface caused the port to be placed in root-inconsistent state when a superior BPDU was received, blocking the port.
C
Loop guard is enabled and the port is in a blocking state due to missing BPDUs.
Why wrong: Loop guard would place the port in a loop-inconsistent state if BPDUs stop being received, not root-inconsistent.
D
The port is in a forwarding state but the VLAN is misconfigured.
Why wrong: The output clearly shows the port is not forwarding; it is in root-inconsistent state.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Root guard is enabled and the port received a superior BPDU, causing it to become root-inconsistent.
Root guard, when enabled on a port, places that port into a root-inconsistent blocking state if it receives a superior BPDU, preventing the switch from becoming the root bridge. This root-inconsistent state stops forwarding traffic, which explains why users in VLAN 10 cannot reach the default gateway. The port remains physically up but is blocked by spanning tree, so normal interface status would not show a down state, making the root-inconsistent state the key indicator.
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 interface is in err-disable state due to BPDU guard.
Why it's wrong here
BPDU guard would place the port in err-disable if a BPDU is received on a PortFast-enabled port.
✓
Root guard is enabled and the port received a superior BPDU, causing it to become root-inconsistent.
Why this is correct
Root guard on the interface caused the port to be placed in root-inconsistent state when a superior BPDU was received, blocking the port.
Clue confirmation
The clue words "best", "which command" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Loop guard is enabled and the port is in a blocking state due to missing BPDUs.
Why it's wrong here
Loop guard would place the port in a loop-inconsistent state if BPDUs stop being received, not root-inconsistent.
✗
The port is in a forwarding state but the VLAN is misconfigured.
Why it's wrong here
The output clearly shows the port is not forwarding; it is in root-inconsistent state.
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.
✓Root guard is enabled and the port received a superior BPDU, causing it to become root-inconsistent.Correct answer▾
Why this is correct
Root guard on the interface caused the port to be placed in root-inconsistent state when a superior BPDU was received, blocking the port.
✗The interface is in err-disable state due to BPDU guard.Wrong answer — click to see why▾
Why this is wrong here
BPDU guard places a port in err-disable state when a BPDU is received on a PortFast-enabled port. However, the question states the interface is down, not err-disabled, and the scenario involves a superior BPDU, which is handled by root guard, not BPDU guard.
Why candidates choose this
Students may confuse BPDU guard with root guard because both involve BPDU protection and can cause ports to block. The term 'err-disable' is commonly associated with BPDU guard, making it a plausible distractor.
✗Loop guard is enabled and the port is in a blocking state due to missing BPDUs.Wrong answer — click to see why▾
Why this is wrong here
Loop guard places a port in a loop-inconsistent state (not root-inconsistent) when BPDUs are no longer received on a nondesignated port. The question indicates the port is down due to a superior BPDU, which is characteristic of root guard, not loop guard.
Why candidates choose this
Both root guard and loop guard are STP enhancement features that can cause ports to block. Students may confuse the conditions: root guard reacts to superior BPDUs, while loop guard reacts to missing BPDUs.
✗The port is in a forwarding state but the VLAN is misconfigured.Wrong answer — click to see why▾
Why this is wrong here
The question states the interface is down, not forwarding. A VLAN misconfiguration would not cause the interface to be down; it would typically result in the interface being up but unable to forward traffic for that VLAN. The output shows a root-inconsistent state, which is a specific STP condition.
Why candidates choose this
Students might think that VLAN misconfiguration could cause connectivity issues, and if they overlook the interface status, they might choose this option. However, the interface being down points to a Layer 1 or STP issue, not a VLAN mismatch.
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
Candidates often confuse root guard with BPDU guard: BPDU guard err-disables a port upon receiving any BPDU on a PortFast port, while root guard responds to superior BPDUs by placing the port in root-inconsistent state, not err-disable.
Trap categories for this question
Command / output trap
The output clearly shows the port is not forwarding; it is in root-inconsistent state.
Detailed technical explanation
How to think about this question
Root guard is configured per interface using the 'spanning-tree guard root' command and forces the port to be a designated port. If a superior BPDU arrives, the port transitions to a root-inconsistent state (shown as 'down' in 'show interfaces' but 'root-inconsistent' in 'show spanning-tree inconsistentports'), blocking all traffic until the superior BPDUs stop. This is distinct from BPDU guard, which err-disables the port, and loop guard, which blocks when BPDUs are lost; root guard specifically protects against rogue root bridge advertisements.
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: Root guard is enabled and the port received a superior BPDU, causing it to become root-inconsistent. — Root guard, when enabled on a port, places that port into a root-inconsistent blocking state if it receives a superior BPDU, preventing the switch from becoming the root bridge. This root-inconsistent state stops forwarding traffic, which explains why users in VLAN 10 cannot reach the default gateway. The port remains physically up but is blocked by spanning tree, so normal interface status would not show a down state, making the root-inconsistent state the key indicator.
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: "best", "which command". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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