The answer is that the port will go into an errdisable state. This occurs because the switch port is configured with port security using the default maximum of one MAC address. When a VoIP phone connects, its own MAC address is learned first, and then the computer plugged into the phone’s built-in switch presents a second, different MAC address, which violates the security policy. By default, any port security violation triggers the errdisable state, which disables the port and requires manual or automatic recovery. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding of how port security interacts with VoIP daisy-chaining—a common trap is assuming the phone’s internal switch is transparent to the switch port. Remember the memory tip: “One MAC, one device; a phone plus a PC means errdisable, not nice.”
N10-009 Network Troubleshooting Practice Question
This N10-009 practice question tests your understanding of network troubleshooting. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: port security limits MAC addresses learned on a switch port.. 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 network technician is troubleshooting a user's inability to connect to the network. The switch port is configured with port security with the default maximum of one MAC address. The user connects a computer and a VoIP phone to the port using the phone's built-in switch. Which of the following will MOST likely occur?
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.
The second MAC address triggers a port security violation, causing the port to be disabled.
B
The phone will work but the computer will not
Why wrong: Port security blocks all traffic from the second MAC, but the switch will typically disable the entire port, not just the computer's traffic.
C
Both devices will work because the phone uses a different MAC
Why wrong: The phone and computer have two unique MAC addresses, exceeding the limit, so a violation occurs.
D
The switch will allow only the first learned MAC and block the second
Why wrong: Port security can operate in restrict or shutdown mode; the default shutdown mode disables the port entirely.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The port will go into errdisable state
With the default port security configuration, the switch port allows only one MAC address. When the VoIP phone connects, its own MAC address is learned first. The computer then connects through the phone's built-in switch, presenting a second MAC address, which violates the security policy. This triggers a security violation, and by default the port is placed into the errdisable state.
Key principle: Port security limits MAC addresses learned on a switch port.
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 clue word "most likely" in the question point toward this answer.
Related concept
Port security limits MAC addresses learned on a switch port.
✗
The phone will work but the computer will not
Why it's wrong here
Port security blocks all traffic from the second MAC, but the switch will typically disable the entire port, not just the computer's traffic.
✗
Both devices will work because the phone uses a different MAC
Why it's wrong here
The phone and computer have two unique MAC addresses, exceeding the limit, so a violation occurs.
✗
The switch will allow only the first learned MAC and block the second
Why it's wrong here
Port security can operate in restrict or shutdown mode; the default shutdown mode disables the port entirely.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume the VoIP phone's built-in switch somehow bypasses port security or that the phone and computer share a single MAC, when in fact each device has its own unique MAC address and the switch enforces the limit per port regardless of device type.
Detailed technical explanation
How to think about this question
Port security with the default maximum of one MAC address uses the 'shutdown' violation mode by default, which places the interface into errdisable state. The switch learns MAC addresses from the source address of incoming frames; the phone's built-in switch forwards the computer's frames, causing the switch to see the computer's MAC. This behavior is independent of any VLAN or voice VLAN configuration—even if the phone uses a separate voice VLAN, the switch still counts the MAC address on the access VLAN unless the port is configured with 'switchport port-security maximum 2' or 'switchport port-security mac-address sticky'.
KKey Concepts to Remember
Port security limits MAC addresses learned on a switch port.
Default port security maximum MAC addresses is often one.
Default port security violation mode is 'shutdown'.
Shutdown mode places the port into an errdisable state.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Port security limits MAC addresses learned on a switch port.
Real-world example
How this comes up in practice
A practitioner preparing for the N10-009 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Port security limits MAC addresses learned on a switch port. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this N10-009 question in full detail.
Review port security limits MAC addresses learned on a switch port., then practise related N10-009 questions on the same topic to reinforce the concept.
Network Troubleshooting — This question tests Network Troubleshooting — Port security limits MAC addresses learned on a switch port..
What is the correct answer to this question?
The correct answer is: The port will go into errdisable state — With the default port security configuration, the switch port allows only one MAC address. When the VoIP phone connects, its own MAC address is learned first. The computer then connects through the phone's built-in switch, presenting a second MAC address, which violates the security policy. This triggers a security violation, and by default the port is placed into the errdisable state.
What should I do if I get this N10-009 question wrong?
Review port security limits MAC addresses learned on a switch port., then practise related N10-009 questions on the same topic to reinforce the concept.
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?
Port security limits MAC addresses learned on a switch port.
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