NCCL Network Topology Lookup Failure (NIC Fusion)
NCCL cannot find a valid path for the network pattern in the discovered PCIe topology. This is typically caused by NIC fusion (NIC split) remapping NIC device IDs during partial-node InfiniBand allocations.
NCCL cannot find a valid path for the network pattern in the discovered PCIe topology.
What this failure is
NCCL Network Topology Lookup Failure (NIC Fusion) is a Distributed Communication failure seen during ML training runs. NCCL cannot find a valid path for the network pattern in the discovered PCIe topology. This is typically caused by NIC fusion (NIC split) remapping NIC device IDs during partial-node InfiniBand allocations. Common tags: Nccl, Topology, Nic Fusion, Network.
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Why it happens (the mechanism)
NIC fusion merges multiple PCIe functions of a single dual-port NIC into one NCCL NET device; on partial-node allocations, the merged topology may be internally inconsistent, and NCCL cannot resolve NET/0. NCCL 2.26.2 has a specific regression in NET/0 resolution for partial-node allocations. Custom topology XML may not account for NIC fusion topology. Taken together, these mechanisms explain why the failure is reproducible, why it tends to surface on specific workloads or scales, and why generic mitigation attempts often fall short without addressing the underlying cause.
What you'll observe
- NCCL fails during initialization with topology lookup error
- NIC fusion (multi-PCIe-function NIC merge) creates inconsistent topology
- NCCL 2.26.2 on partial-node IB allocations is especially affected
- Manual NCCL_TOPO_FILE may have incomplete or incorrect entries
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| Could not find NET with id 0 during NCCL initialization | NIC fusion merges multiple PCIe functions of a single dual-port NIC into one NCCL NET device; on partial-node allocations, the merged topology may be internally inconsistent, and NCCL cannot resolve NET/0 |
| Could not find a path for pattern during NCCL initialization | NCCL 2.26.2 has a specific regression in NET/0 resolution for partial-node allocations |
| NCCL fails before any collective operation starts (initialization-only failure) | Custom topology XML may not account for NIC fusion topology |
| Error only occurs on multi-GPU/multi-node jobs, not single-GPU | NIC fusion merges multiple PCIe functions of a single dual-port NIC into one NCCL NET device; on partial-node allocations, the merged topology may be internally inconsistent, and NCCL cannot resolve NET/0 |
Which systems are affected
- NVIDIA H100/H200 with InfiniBand NDR 400G (dual-port NICs with PCIe bifurcation)
- Partial-node Slurm allocations where not all GPUs/NICs are assigned
- NCCL 2.26.2 on systems with NIC fusion topology
- Systems with NCCL_TOPO_FILE custom XML configurations
How to confirm this is the problem
Use this checklist to test the hypothesis against a small reproduction. No single line proves the root cause, so preserve the preceding events and compare one variable at a time.
- ✓Reproduce the failure from a clean checkpoint/seed: the symptom must appear without warm-up state from a previous run.
- ✓Verified signal present: Could not find NET with id 0 during NCCL initialization
- ✓Verified signal present: Could not find a path for pattern during NCCL initialization
- ✓Verified signal present: NCCL fails before any collective operation starts (initialization-only failure)
- ✓Verified signal present: Error only occurs on multi-GPU/multi-node jobs, not single-GPU
- ✓A targeted fix from the "How to fix it" section eliminates or substantially reduces the symptom within one validation pass.
The fix and the prevention pattern
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Install the free VS Code extensionNCCL errors in context
NCCL is where a distributed job reports failure, which is not the same as where it failed. The hub lists every common NCCL error next to what it actually indicates, and the environment variables that tell them apart.
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Root cause
- NIC fusion merges multiple PCIe functions of a single dual-port NIC into one NCCL NET device; on partial-node allocations, the merged topology may be internally inconsistent, and NCCL cannot resolve NET/0
- NCCL 2.26.2 has a specific regression in NET/0 resolution for partial-node allocations
- Custom topology XML may not account for NIC fusion topology
The fix and how to prevent it
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