CCNA Switching and Network Access Practice Question
A network administrator notices that a switch port connecting to an end-user workstation is flapping between up and down states. The port is configured with PortFast and BPDU Guard. The workstation is replaced with a small unmanaged switch that begins sending BPDUs. What is the expected result on the switch port?
⚠ Common exam trap
The trap here is believing BPDU Guard blocks the port in STP terms, when it actually places the interface into the err-disabled state.
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
✓
The port enters the err-disabled state because BPDU Guard detects the BPDU on a PortFast-enabled port.
BPDU Guard protects PortFast ports by err-disabling them when a BPDU is received. A PortFast port is meant for end hosts, so BPDUs indicate an unauthorized switch, and the guard action prevents a potential Layer 2 loop. The port becomes err-disabled rather than blocking, and it stays down until an administrator recovers it or err-disable recovery is configured.
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 port enters the err-disabled state because BPDU Guard detects the BPDU on a PortFast-enabled port.
Why this is correct
BPDU Guard is designed to shut down a PortFast-enabled port if it receives a BPDU, which indicates an unauthorized switch is connected. When the unmanaged switch sends BPDUs, the port violates the guard condition and transitions to err-disabled, protecting the spanning-tree topology from potential loops.
- ✗
The port transitions to the blocking state but remains administratively up.
Why it's wrong here
BPDU Guard does not place the port in the spanning-tree blocking state; it err-disables the interface, which is a different operational state. A blocking port is still administratively enabled and participates in STP, whereas an err-disabled port is shut down by the switch and does not pass any traffic until it is recovered.
- ✗
The port remains in the forwarding state and simply logs a syslog message about the BPDU.
Why it's wrong here
BPDU Guard does not merely log; it takes the port down to err-disabled when a BPDU is received on a PortFast port. Logging alone would leave the topology exposed to a loop if the connected device is a switch. Therefore the port would not stay in forwarding, and the described behavior does not match the guard feature.
- ✗
The port automatically becomes a trunk and negotiates with the unmanaged switch.
Why it's wrong here
PortFast and BPDU Guard do not trigger trunk negotiation, and an unmanaged switch does not run DTP in a way that would form a trunk with the access port. The presence of BPDUs is treated as a violation, so the port is disabled rather than converted into a trunk link.
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Key term
Err-disabled
Err-disabled is a switch port state that occurs when the switch detects a critical error on that port and automatically shuts it down to protect the network.
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 and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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