Question 88 of 144
SK0-005 troubleshooting Practice Question
An administrator notices that a Linux server's /var/log/messages file is filled with repeated "eth0: link up" and "eth0: link down" entries every few seconds. The server is connected to a managed switch. Which of the following is the most likely cause?
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 switch port is experiencing a flapping MAC address
Option D is correct because MAC address flapping on a switch port causes the link state to repeatedly change. Option A would cause performance issues but not constant link state changes. Option B might cause instability but is not the typical cause of flapping. Option C would block traffic but not cause the physical link to go up and down.
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 switch port is configured for auto-negotiation while the NIC is set to 1000/full
Why it's wrong here
A speed/duplex mismatch leads to CRC errors and poor performance but does not cause the physical link to drop repeatedly.
- ✗
The NIC driver is outdated
Why it's wrong here
An outdated driver may cause sporadic issues but is less likely to produce such a clear pattern of repeated link state changes.
- ✗
Spanning Tree Protocol is blocking the port
Why it's wrong here
STP blocking prevents data forwarding but the physical link remains up; it would not generate link up/down messages.
- ✓
The switch port is experiencing a flapping MAC address
Why this is correct
MAC flapping causes the switch to detect constant changes, resulting in the physical link going up and down rapidly.
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 |
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Last reviewed: Jul 26, 2026
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