CCNA Switching and Network Access Practice Question
After connecting a new switch to interface GigabitEthernet1/0/1 on a distribution switch, a network engineer notices that the interface is in err-disable state. The engineer checks the configuration and finds that spanning-tree portfast and spanning-tree bpduguard enable are applied to the interface. What is the most likely cause of the err-disable state?
⚠ Common exam trap
Cisco often tests the misconception that BPDU Guard alone causes err-disable, but the trap here is that PortFast must be enabled for BPDU Guard to trigger err-disable on a port receiving BPDUs from another switch.
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
✓
Spanning-tree PortFast is enabled on a port that connects to another switch.
PortFast is designed for end-host ports that should not receive BPDUs. When PortFast is enabled on a port connecting to another switch, the switch will immediately transition the port to forwarding state, but if it then receives a BPDU from the connected switch, BPDU Guard will error-disable the port. This is the most common cause of err-disable state when both PortFast and BPDU Guard are configured on an inter-switch link.
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 was incorrectly enabled on a port that should be a trunk link.
Why it's wrong here
BPDU Guard can be enabled on any port, not just access ports. The err-disable occurred because BPDU Guard received a BPDU while PortFast was enabled on an inter-switch link. If the port were a trunk without PortFast, BPDU Guard would not be triggered unless globally forced, but the question states both features were present. The port being an access or trunk does not directly cause the err-disable.
- ✗
The connected switch is sending BPDUs with a lower bridge priority.
Why it's wrong here
BPDU Guard triggers on any BPDU, regardless of bridge priority or root bridge election. A lower priority simply influences STP topology; it does not cause an err-disable condition. The trigger is the receipt of a BPDU, not its contents.
- ✓
Spanning-tree PortFast is enabled on a port that connects to another switch.
Why this is correct
PortFast skips the listening and learning STP states and is designed for end hosts. When combined with BPDU Guard, the switch was instructed to disable the port upon receiving any BPDU. The downstream switch naturally sends BPDUs, causing BPDU Guard to react and place the port in err-disable. Removing PortFast (and leaving BPDU Guard alone, or disabling BPDU Guard on that link) would resolve the issue.
- ✗
The port is configured as an access port, but should be a trunk.
Why it's wrong here
An access/trunk mismatch would cause VLAN-related problems, but it would not lead to an err-disable triggered by BPDU Guard. Switches still send BPDUs on access ports by default, so the BPDU Guard reaction is independent of the port mode. The issue here is PortFast, not the switchport mode.
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.
✓Spanning-tree PortFast is enabled on a port that connects to another switch.Correct answer▾
Why this is correct
PortFast skips the listening and learning STP states and is designed for end hosts. When combined with BPDU Guard, the switch was instructed to disable the port upon receiving any BPDU. The downstream switch naturally sends BPDUs, causing BPDU Guard to react and place the port in err-disable. Removing PortFast (and leaving BPDU Guard alone, or disabling BPDU Guard on that link) would resolve the issue.
✗BPDU Guard was incorrectly enabled on a port that should be a trunk link.Wrong answer — click to see why▾
Why this is wrong here
Candidates often associate BPDU Guard exclusively with access ports and assume configuring it on a trunk is itself a misconfiguration, overlooking that PortFast is the real culprit.
✗The connected switch is sending BPDUs with a lower bridge priority.Wrong answer — click to see why▾
Why this is wrong here
Students may confuse root bridge placement with BPDU Guard operation, thinking that a BPDU from a superior switch might cause a port to be disabled, when in fact BPDU Guard is content-agnostic.
✗The port is configured as an access port, but should be a trunk.Wrong answer — click to see why▾
Why this is wrong here
Candidates may think that because a link between switches should be a trunk, the access mode misconfiguration is the root cause. However, they miss the fact that BPDU Guard acts on the BPDU regardless of the port mode, and the real misconfiguration is PortFast.
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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Related to this question
Learn chapter
Configuring Switch Ports for Desktops, VoIP Phones, APs, IoT, and Virtualized Hosts
Key term
BPDU Guard
BPDU Guard is a Cisco switch feature that protects the network from unauthorized devices by disabling a port if it receives a Bridge Protocol Data Unit (BPDU).
Key term
PortFast
PortFast is a Cisco switch feature that immediately brings a port into the forwarding state, bypassing the normal Spanning Tree Protocol (STP) listening and learning phases, so that devices connected to that port can start communicating right away.
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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.