CCNA Switching and Network Access Practice Question
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?
⚠ Common exam trap
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.
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
✓
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.
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 is a PortFast companion feature that immediately err-disables a port when any BPDU is received, typically on access ports facing end hosts. The exhibit's 'root-inconsistent' state is a distinct STP condition that appears in 'show spanning-tree inconsistentports'; BPDU Guard would instead log an err-disable event and place the port in 'errdisable' state, not 'root-inconsistent'. Therefore, this option misreads the output, which clearly shows a root-inconsistent inconsistency rather than an err-disable condition.
- ✗
Loop Guard is enabled, and the port has stopped receiving BPDUs, leading to a loop-inconsistent state.
Why it's wrong here
Loop Guard protects against unidirectional link failures by moving a designated port into the 'loop-inconsistent' state when BPDUs stop arriving; it is designed to prevent alternate or root ports from becoming designated and creating loops. The output explicitly shows 'root-inconsistent', which is generated by Root Guard, not Loop Guard. Additionally, Loop Guard's trigger is the absence of BPDUs, whereas root-inconsistent occurs because a superior BPDU was received—exactly the opposite condition, making this option incorrect.
- ✓
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 is the mechanism that enforces the root bridge position by transitioning a port to the 'root-inconsistent' state when a superior BPDU (lower bridge ID) arrives, effectively blocking the port. Because the new access switch is sending superior BPDUs, the port has been placed in a blocked state to prevent it from becoming the root port and usurping the root bridge. The 'show spanning-tree inconsistentports' output confirms this condition, and the port will automatically recover once the superior BPDUs cease, which is consistent with the exhibit.
- ✗
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 is the mechanism that enforces the root bridge position by transitioning a port to the 'root-inconsistent' state when a superior BPDU (lower bridge ID) arrives, effectively blocking the port. Because the new access switch is sending superior BPDUs, the port has been placed in a blocked state to prevent it from becoming the root port and usurping the root bridge. The 'show spanning-tree inconsistentports' output confirms this condition, and the port will automatically recover once the superior BPDUs cease, which is consistent with the exhibit.
✗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?”
Visual reference
Go deeper
Related to this question
Learn chapter
Configuring Switch Ports for Desktops, VoIP Phones, APs, IoT, and Virtualized Hosts
Key term
Bridge priority
Bridge priority is a numerical value used in Spanning Tree Protocol to determine which switch becomes the root bridge in a network.
Key term
Root Guard
Root Guard is a Spanning Tree Protocol (STP) feature that protects the root bridge placement on a network port to prevent unauthorized switches from becoming the root bridge.
About these practice questions
This 200-301 question is part of Courseiva's 1,389-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.