- A
Replace the Ethernet cable.
Why wrong: The link light is green, indicating a good physical connection. Replacing the cable is unlikely to resolve the issue.
- B
Check if the switch port is configured with Port Security that is blocking the MAC address.
Port Security can limit the number of MAC addresses on a port or block specific MACs, preventing DHCP requests from being processed. This is a common cause of DHCP failures on otherwise functional ports.
- C
Reboot the DHCP server.
Why wrong: Other devices are working, so the DHCP server is functional. Rebooting it would be disruptive and unnecessary.
- D
Set a static IP address on the computer.
Why wrong: While this would bypass DHCP, it does not solve the underlying issue and may cause IP conflicts. Troubleshooting should focus on why DHCP is failing.
DHCP Not Working on Switch Port with Port Security
This 220-1201 practice question tests your understanding of common networking hardware. 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 a user's computer cannot obtain an IP address via DHCP. The computer is connected to a switch, and other devices on the same switch work fine. The technician checks the switch and sees that the port is up and the link light is green. What should the technician check next?
Quick Answer
The correct next step is to check if the switch port is configured with Port Security that is blocking the MAC address. When DHCP is not working on a switch port with port security, the switch drops frames from any unknown MAC address, preventing the DHCP discover broadcast from reaching the server. Since the link light is green, the physical layer is fine, so the issue lies in Layer 2 access control. On the CompTIA A+ Core 1 220-1201 exam, this tests your ability to isolate DHCP failures beyond the usual suspects like a bad cable or disabled port. A common trap is to immediately blame the DHCP server or IP configuration, but when other devices on the same switch work, the problem is almost certainly port-level. Remember the mnemonic “Link green, no IP? Check security policy.”
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
Check if the switch port is configured with Port Security that is blocking the MAC address.
Since the switch port is up and other devices on the same switch obtain IP addresses correctly, the DHCP server and the general network path are functional. The most likely cause is a Layer 2 security feature, such as Port Security, which can be configured to block unknown or specific MAC addresses, preventing the computer from communicating with the DHCP server even though the physical link is active.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Replace the Ethernet cable.
Why it's wrong here
The link light is green, indicating a good physical connection. Replacing the cable is unlikely to resolve the issue.
- ✓
Check if the switch port is configured with Port Security that is blocking the MAC address.
Why this is correct
Port Security can limit the number of MAC addresses on a port or block specific MACs, preventing DHCP requests from being processed. This is a common cause of DHCP failures on otherwise functional ports.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Reboot the DHCP server.
Why it's wrong here
Other devices are working, so the DHCP server is functional. Rebooting it would be disruptive and unnecessary.
- ✗
Set a static IP address on the computer.
Why it's wrong here
While this would bypass DHCP, it does not solve the underlying issue and may cause IP conflicts. Troubleshooting should focus on why DHCP is failing.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a green link light guarantees full Layer 2 connectivity, when in fact security features like Port Security can block traffic at the MAC layer while the physical layer remains operational.
Detailed technical explanation
How to think about this question
Port Security operates by associating specific MAC addresses with a switch port; if the port is configured with a maximum MAC address count or a sticky MAC list, an unauthorized MAC will be placed into an err-disabled or restricted state, dropping all frames. This can be verified with the 'show port-security interface' command, which displays the violation count and the action taken (shutdown, restrict, or protect). In a real-world scenario, a technician might also check for DHCP snooping, which can drop DHCP offers from untrusted ports, but Port Security is the more direct cause when the link is up but no IP is obtained.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
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
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 220-1201 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. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Visual reference
Quick reference
Access Control Model Comparison
| Model | Acronym | Who Controls Access? | Best For |
|---|---|---|---|
| Discretionary Access Control | DAC | Resource owner | Small teams, file shares |
| Mandatory Access Control | MAC | System / security labels | Classified govt / military |
| Role-Based Access Control | RBAC | Administrator (via roles) | Enterprise environments |
| Attribute-Based Access Control | ABAC | Policy engine (user + resource attributes) | Fine-grained, dynamic policies |
| Rule-Based Access Control | RuBAC | System rules / ACLs | Firewall rules, network ACLs |
What to study next
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FAQ
Questions learners often ask
What does this 220-1201 question test?
Common Networking Hardware — This question tests Common Networking Hardware — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Check if the switch port is configured with Port Security that is blocking the MAC address. — Since the switch port is up and other devices on the same switch obtain IP addresses correctly, the DHCP server and the general network path are functional. The most likely cause is a Layer 2 security feature, such as Port Security, which can be configured to block unknown or specific MAC addresses, preventing the computer from communicating with the DHCP server even though the physical link is active.
What should I do if I get this 220-1201 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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