The answer is a port security violation, which is the most likely cause of an err-disabled switch port in this scenario. When a switch detects an unauthorized MAC address or exceeds the allowed MAC address limit on a port, it automatically places the port into an err-disabled state to prevent unauthorized access or network loops. This state can also be triggered by other protective mechanisms like BPDU guard, but on the CompTIA A+ Core 1 220-1201 exam, port security violations are the classic cause tested for err-disabled ports. A common trap is confusing this with a physical cable fault—while a bad cable can cause errors, it typically results in a down or flapping port, not an err-disabled status. To remember, think of “security first”: the switch disables the port to protect the network, not because the link is broken.
220-1101 Network Configuration Concepts Practice Question
This 220-1201 practice question tests your understanding of network configuration concepts. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
A technician is troubleshooting a network where users on one floor cannot access a server on another floor. Both floors are connected via a single fiber optic link. The technician checks the link and sees the switch port shows 'err-disabled' status. What is the most likely cause of this state?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Port security has been violated
An 'err-disabled' state on a switch port typically occurs due to a port security violation, such as too many MAC addresses detected, or a loop detection mechanism like BPDU guard. This indicates a security or configuration issue that disabled the port automatically.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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 fiber cable is damaged
Why it's wrong here
A damaged cable would cause the port to show 'down' or 'notconnect', not 'err-disabled', which is a software-disabled state.
✗
A duplex mismatch exists between the switches
Why it's wrong here
Duplex mismatch causes high error rates and poor performance but typically results in 'down' or 'flapping' ports, not 'err-disabled'.
✓
Port security has been violated
Why this is correct
Port security violations, such as exceeding the maximum MAC addresses, cause the switch to disable the port and place it in err-disabled state.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Access ports place end devices into a single VLAN.
✗
The VLAN configuration is incorrect
Why it's wrong here
Incorrect VLAN config would allow the port to be up but traffic may not pass; it would not cause err-disabled state.
Common exam traps
Common exam trap: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Trap categories for this question
Command / output trap
A damaged cable would cause the port to show 'down' or 'notconnect', not 'err-disabled', which is a software-disabled state.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
Access ports place end devices into a single VLAN.
Trunk ports carry multiple VLANs between switches.
Allowed VLAN lists decide which VLANs can cross a trunk.
Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
→Use show vlan brief to verify access VLANs.
→Use show interfaces trunk to verify trunk state and allowed VLANs.
→Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 220-1201 question in full detail.
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 220-1201 questions on switching, trunking, and access-port configuration.
Network Configuration Concepts — This question tests Network Configuration Concepts — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: Port security has been violated — An 'err-disabled' state on a switch port typically occurs due to a port security violation, such as too many MAC addresses detected, or a loop detection mechanism like BPDU guard. This indicates a security or configuration issue that disabled the port automatically.
What should I do if I get this 220-1201 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 220-1201 questions on switching, trunking, and access-port configuration.
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?
Access ports place end devices into a single VLAN.
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