Question 675 of 1,389
CCNA Switching and Network Access Practice Question
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?
⚠ Common exam trap
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.
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 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.
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 is an interface feature that transitions a PortFast-enabled port to err-disable state if it receives any BPDU, because such a port should only connect to end hosts. In this scenario, the port is not in err-disable; it is shown in a root-inconsistent STP state, which is an administrative reaction to receiving a superior BPDU rather than a hard shutdown. Additionally, BPDU guard would not produce a 'root-inconsistent' status in STP output.
- ✓
Root guard is enabled and the port received a superior BPDU, causing it to become root-inconsistent.
Why this is correct
Root guard is correctly the cause: when a port configured with root guard receives a superior BPDU (i.e., a BPDU that would make the local switch root or change the root bridge), the port is moved to a root-inconsistent state. This blocks all traffic on the port to protect the existing root bridge and prevent a rogue switch from hijacking the spanning tree. The symptom matches exactly—the port is administratively placed in this state, not in err-disable or loop-inconsistent.
- ✗
Loop guard is enabled and the port is in a blocking state due to missing BPDUs.
Why it's wrong here
Loop guard is a separate STP protection mechanism that reacts to the absence of BPDUs, not to superior BPDUs. If BPDUs stop arriving on a port that was designated or root, loop guard places it into a loop-inconsistent state (blocking) to prevent alternate paths from creating a loop. The output here indicates a root-inconsistent state, which is specifically triggered by root guard, not loop guard.
- ✗
The port is in a forwarding state but the VLAN is misconfigured.
Why it's wrong here
The interface is not in a forwarding state; the STP state is root-inconsistent, which is a special state that blocks traffic, similar to listening/blocking. A VLAN misconfiguration might make the port non-functional or place it in a different VLAN, but it would not directly cause the port to enter a root-inconsistent state. Root-inconsistent is a deliberate STP state triggered by root guard, so VLAN misconfiguration is not the underlying reason.
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 is correctly the cause: when a port configured with root guard receives a superior BPDU (i.e., a BPDU that would make the local switch root or change the root bridge), the port is moved to a root-inconsistent state. This blocks all traffic on the port to protect the existing root bridge and prevent a rogue switch from hijacking the spanning tree. The symptom matches exactly—the port is administratively placed in this state, not in err-disable or loop-inconsistent.
✗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?”
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Last reviewed: Jun 11, 2026
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