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NIC Degradation (Link Speed Low / NIC Lost / GID Error)

A network interface degrades. Negotiating a lower link speed, disappearing, or reporting GID errors. Silently throttling RDMA collective throughput across the job.

Quick answer

A network interface degrades. Negotiating a lower link speed, disappearing, or reporting GID errors. Silently throttling RDMA collective throughput across the job.

Infrastructure#nic#link-speed#degradation#rdma#infrastructure

What this failure is

NIC Degradation (Link Speed Low / NIC Lost / GID Error) is a Infrastructure failure seen during ML training runs. A network interface degrades. Negotiating a lower link speed, disappearing, or reporting GID errors. Silently throttling RDMA collective throughput across the job. Common tags: Nic, Link Speed, Degradation, Rdma.

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Why it happens (the mechanism)

Cable/transceiver degradation forcing a lower negotiated speed. Firmware or PCIe issue dropping the NIC. GID table / RoCE addressing errors. 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

  • Collective throughput drops without an outright failure
  • A NIC negotiates below its rated speed
  • Intermittent NIC disappearance or GID errors

Common symptoms and what they mean

SymptomWhy it happens
NIC Link Speed low (e.g. 100G port at 25G)Cable/transceiver degradation forcing a lower negotiated speed
NIC Lost / device disappears from the fabricFirmware or PCIe issue dropping the NIC
Rnic Gid Error in logsGID table / RoCE addressing errors

Which systems are affected

  • RoCE/InfiniBand clusters
  • Bonded multi-NIC nodes
  • Long-running production training

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: NIC Link Speed low (e.g. 100G port at 25G)
  • ✓Verified signal present: NIC Lost / device disappears from the fabric
  • ✓Verified signal present: Rnic Gid Error in logs
  • ✓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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Related failures to investigate next

Lambda Bgp Route Withdrawal Region Outage often co-occurs with this failure because both share underlying assumptions about infrastructure; if you fix this entry but still see drift, validate the related primary cause next. lambda bgp route withdrawal region outage →
Kubernetes Gpu Pod Pending often co-occurs with this failure because both share underlying assumptions about infrastructure; if you fix this entry but still see drift, validate the related neighboring cause next. kubernetes gpu pod pending →
Cloud Storage Throttling often co-occurs with this failure because both share underlying assumptions about infrastructure; if you fix this entry but still see drift, validate the related neighboring cause next. cloud storage throttling →

Root cause

  • Cable/transceiver degradation forcing a lower negotiated speed
  • Firmware or PCIe issue dropping the NIC
  • GID table / RoCE addressing errors

The fix and how to prevent it

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References

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