CV0-004 Troubleshooting Practice Question
A cloud administrator is troubleshooting a Linux virtual machine (VM) in a public cloud that is experiencing packet loss when communicating with another VM in the same subnet. The administrator runs 'ip -s link show eth0' and observes a high number of dropped packets on the receive side. The VM's CPU utilization is low, and the network interface is a paravirtualized driver (virtio). Which of the following is the MOST likely cause of the dropped packets?
⚠ Common exam trap
The trap here is assuming that packet loss in the cloud is due to physical network problems, when in virtualized environments it is often a driver buffer or configuration issue.
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 receive ring buffer on the network interface is too small, causing packets to be dropped when the buffer overflows.
The high number of receive drops on the virtio interface, combined with low CPU utilization, points to receive ring buffer exhaustion. The virtio driver uses a ring buffer to hold incoming packets; if the buffer is too small for the traffic rate, packets are dropped. Increasing the ring buffer size with ethtool -G eth0 rx <size> can resolve the issue. Physical network issues and firewall drops are less likely in a cloud environment and would not manifest as interface-level receive drops.
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 receive ring buffer on the network interface is too small, causing packets to be dropped when the buffer overflows.
Why this is correct
A high number of receive drops on a virtio interface often indicates that the receive ring buffer is full. When packets arrive faster than the guest can process them, the buffer overflows and packets are dropped. This is common in virtualized environments where the virtio driver's ring size may be default and insufficient for high throughput. Increasing the ring buffer size using ethtool -G can mitigate the issue, and low CPU utilization suggests the guest is not overwhelmed, pointing to buffer capacity.
- ✗
The VM's firewall is dropping packets due to a misconfigured rule.
Why it's wrong here
Firewall drops would typically be logged and counted separately, not as interface receive drops. The 'ip -s link show' command reports drops at the driver level, such as buffer overflows or errors, not firewall drops. Firewall rules would cause packets to be dropped after they are received and processed by the network stack, and those drops would not increment the interface's rx_dropped counter. Thus, a firewall misconfiguration is not the cause here.
- ✗
There is a physical network issue, such as a faulty cable or switch port.
Why it's wrong here
In a public cloud, the physical network is managed by the provider and is highly reliable. The VMs are in the same subnet, so traffic likely stays within the virtual network. Physical cable or switch port issues are not applicable in cloud environments; the provider abstracts the physical layer. The symptom of drops on the receive side of a virtio interface is more consistent with virtualized driver buffer limitations than physical faults.
- ✗
The VM's CPU is not allocating enough cycles to process network interrupts.
Why it's wrong here
Low CPU utilization indicates the CPU is not saturated, so it is unlikely that interrupt processing is starved. If CPU were the bottleneck, you would see high CPU usage, especially in softirq context. The dropped packets are on the receive side, and with low CPU, the issue is more likely buffer-related. While interrupt handling can cause drops, the low CPU utilization makes this an unlikely cause in this scenario.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official CompTIA exam blueprint
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