The answer is that Root Guard is configured on the distribution switch port, and the new access switch is sending superior BPDUs, triggering a root-inconsistent state. This occurs because Root Guard, when enabled on a port, actively monitors incoming BPDUs; if it receives a BPDU that is superior to the current root bridge’s BPDU, it immediately places the port into a root-inconsistent (blocking) state to prevent the new switch from usurping the root bridge role. On the CCNA 200-301 v2 exam, this scenario tests your understanding of spanning-tree protection mechanisms—a common trap is confusing Root Guard with BPDU Guard, but remember that Root Guard reacts to superior BPDUs, while BPDU Guard reacts to any BPDU on a PortFast-enabled port. A useful memory tip: think of Root Guard as the “bodyguard” that blocks any port trying to become the new root, keeping the current root safe.
CCNA Switching and Network Access Practice Question
This 200-301 practice question tests your understanding of switching and network access. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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
SW-DIST# show spanning-tree inconsistentports
Name Interface Inconsistency
-------------------- ------------------------ ------------------
VLAN10 GigabitEthernet1/0/1 Root Inconsistent
An administrator connects a new access-layer switch to a distribution switch. The link comes up but remains in a blocking state and does not forward frames. The administrator issues the show command shown in the exhibit. What is the most likely reason the link is blocked?
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.
SW-DIST# show spanning-tree inconsistentports
Name Interface Inconsistency
-------------------- ------------------------ ------------------
VLAN10 GigabitEthernet1/0/1 Root Inconsistent
A
BPDU Guard is enabled on the port, and the new switch's BPDUs caused the port to enter err-disable state.
Why wrong: BPDU Guard causes a port to go into err-disable state, not root-inconsistent. The exhibit shows a root-inconsistent inconsistency, not err-disable.
B
Loop Guard is enabled, and the port has stopped receiving BPDUs, leading to a loop-inconsistent state.
Why wrong: Loop Guard places a port into loop-inconsistent state when BPDUs are no longer received on a designated port. The output shows root-inconsistent, not loop-inconsistent.
C
Root Guard is configured on the port, and the new access switch is sending superior BPDUs, triggering a root-inconsistent state.
Root Guard intentionally places a port into root-inconsistent (blocking) state when it receives a superior BPDU, preventing an unwanted switch from becoming the root bridge. The show spanning-tree inconsistentports output confirms a root-inconsistent condition.
D
The port is configured with PortFast, and the new switch's BPDU triggered a loop, causing the port to err-disable.
Why wrong: PortFast alone does not block a port upon receiving a BPDU; it only transitions the port directly to forwarding. If BPDU Guard were also enabled, the port would err-disable, but the exhibit shows a root-inconsistent state, not err-disable.
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 configured on the port, and the new access switch is sending superior BPDUs, triggering a root-inconsistent state.
The link is blocked because Root Guard is configured on the distribution switch port. When the new access-layer switch sends superior BPDUs (with a lower bridge priority), Root Guard transitions the port to a root-inconsistent (blocking) state to protect the current root bridge from being usurped. This prevents the new switch from becoming the root bridge, which would disrupt the spanning-tree topology.
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.
✗
BPDU Guard is enabled on the port, and the new switch's BPDUs caused the port to enter err-disable state.
Why it's wrong here
BPDU Guard causes a port to go into err-disable state, not root-inconsistent. The exhibit shows a root-inconsistent inconsistency, not err-disable.
✗
Loop Guard is enabled, and the port has stopped receiving BPDUs, leading to a loop-inconsistent state.
Why it's wrong here
Loop Guard places a port into loop-inconsistent state when BPDUs are no longer received on a designated port. The output shows root-inconsistent, not loop-inconsistent.
✓
Root Guard is configured on the port, and the new access switch is sending superior BPDUs, triggering a root-inconsistent state.
Why this is correct
Root Guard intentionally places a port into root-inconsistent (blocking) state when it receives a superior BPDU, preventing an unwanted switch from becoming the root bridge. The show spanning-tree inconsistentports output confirms a root-inconsistent condition.
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.
✗
The port is configured with PortFast, and the new switch's BPDU triggered a loop, causing the port to err-disable.
Why it's wrong here
PortFast alone does not block a port upon receiving a BPDU; it only transitions the port directly to forwarding. If BPDU Guard were also enabled, the port would err-disable, but the exhibit shows a root-inconsistent state, not err-disable.
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 configured on the port, and the new access switch is sending superior BPDUs, triggering a root-inconsistent state.Correct answer▾
Why this is correct
Root Guard intentionally places a port into root-inconsistent (blocking) state when it receives a superior BPDU, preventing an unwanted switch from becoming the root bridge. The show spanning-tree inconsistentports output confirms a root-inconsistent condition.
✗BPDU Guard is enabled on the port, and the new switch's BPDUs caused the port to enter err-disable state.Wrong answer — click to see why▾
Why this is wrong here
BPDU Guard results in err-disable, which is not reflected in the show spanning-tree inconsistentports output. The exhibit clearly shows Root Inconsistent, indicating Root Guard, not BPDU Guard.
✗Loop Guard is enabled, and the port has stopped receiving BPDUs, leading to a loop-inconsistent state.Wrong answer — click to see why▾
Why this is wrong here
Loop Guard creates a loop-inconsistent listing, not root-inconsistent. The command output explicitly indicates Root Inconsistent, ruling out Loop Guard.
✗The port is configured with PortFast, and the new switch's BPDU triggered a loop, causing the port to err-disable.Wrong answer — click to see why▾
Why this is wrong here
Without BPDU Guard, PortFast does not react to BPDUs with a blocking state. The exhibit's root-inconsistent inconsistency is specific to Root Guard, not PortFast or 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 distinction between err-disable states (BPDU Guard) and blocking states (Root Guard, Loop Guard), so the trap here is assuming any BPDU-related protection causes err-disable, when Root Guard specifically causes a blocking state without err-disable.
Trap categories for this question
Command / output trap
BPDU Guard causes a port to go into err-disable state, not root-inconsistent. The exhibit shows a root-inconsistent inconsistency, not err-disable.
Detailed technical explanation
How to think about this question
Root Guard is configured per interface using the 'spanning-tree guard root' command. It forces the port to be a designated port; if the port receives a superior BPDU (lower bridge ID), it moves to a root-inconsistent state (blocking) until the superior BPDUs stop. This is distinct from BPDU Guard, which err-disables the port, and Loop Guard, which blocks when BPDUs are absent. In real-world networks, Root Guard is often deployed on access ports to prevent unauthorized switches from becoming the root bridge.
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.
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 configured on the port, and the new access switch is sending superior BPDUs, triggering a root-inconsistent state. — The link is blocked because Root Guard is configured on the distribution switch port. When the new access-layer switch sends superior BPDUs (with a lower bridge priority), Root Guard transitions the port to a root-inconsistent (blocking) state to protect the current root bridge from being usurped. This prevents the new switch from becoming the root bridge, which would disrupt the spanning-tree topology.
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.
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