NCP-AIO Administration Practice Question
An administrator is troubleshooting a performance degradation in a multi-node NVIDIA NCCL-based training job. The job spans four DGX nodes connected via InfiniBand. The administrator suspects that NCCL is not using the optimal network path. Which action should the administrator take to verify and enforce the use of GPUDirect RDMA for inter-node communication?
⚠ Common exam trap
The trap here is misunderstanding NCCL_NET_GDR_LEVEL: a value of 0 disables GPUDirect RDMA, while higher values enable it based on topology distance.
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
✓
Set the environment variable NCCL_DEBUG=INFO and inspect the logs for messages indicating that GPUDirect RDMA is enabled.
To verify and enforce GPUDirect RDMA usage, the administrator should enable NCCL debug logging with NCCL_DEBUG=INFO and examine the logs for transport information. This will show if GDR is active and if any fallbacks occur. Checking topology with nvidia-smi topo -m is useful but does not confirm active GDR. Setting NCCL_NET_GDR_LEVEL=0 would disable GDR, and switching to RoCEv2 is irrelevant.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Configure the InfiniBand fabric to use RoCEv2 instead of InfiniBand native protocol to improve compatibility with GPUDirect RDMA.
Why it's wrong here
GPUDirect RDMA works over both InfiniBand and RoCE, but switching to RoCEv2 is not necessary and could introduce additional configuration complexity. The native InfiniBand protocol is typically preferred for its low latency and high throughput. This action does not verify or enforce GDR usage and may degrade performance if not properly configured.
- ✗
Set NCCL_NET_GDR_LEVEL=0 to force NCCL to use GPUDirect RDMA for all inter-node communication.
Why it's wrong here
NCCL_NET_GDR_LEVEL controls the threshold for using GPUDirect RDMA based on the distance between GPU and NIC. Setting it to 0 does not force GDR; it actually disables GDR because the level represents the maximum allowed distance. To enable GDR, the level should be set to a value that matches or exceeds the actual topology distance, such as 3 or 4. Thus, this action would likely disable GDR, not enforce it.
- ✓
Set the environment variable NCCL_DEBUG=INFO and inspect the logs for messages indicating that GPUDirect RDMA is enabled.
Why this is correct
Setting NCCL_DEBUG=INFO generates detailed logs that show which transports NCCL uses, including whether GPUDirect RDMA is active. The logs will indicate if the network plugin is using RDMA and if there are any fallbacks to slower paths. This is the standard method to verify NCCL's behavior and diagnose performance issues related to network communication.
- ✗
Run nvidia-smi topo -m on each node to check the GPU-to-NIC affinity and ensure that GPUs and InfiniBand adapters are connected via PCIe switches.
Why it's wrong here
nvidia-smi topo -m shows the topology between GPUs and NICs, which is important for optimal performance, but it does not verify whether NCCL is actually using GPUDirect RDMA. It only shows the physical connectivity. While useful for diagnosing potential bottlenecks, it does not confirm the active transport or enforce its use.
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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 NVIDIA exam blueprint
This NCP-AIO practice question is part of Courseiva's free NVIDIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the NCP-AIO exam.