The Conclusion First
> "RoCEv2 is always cheaper than IB" is true. "RoCEv2 is always slower than IB" is false.
What makes the difference is the ops team's RDMA experience and the workload's latency sensitivity.
The Three-Axis Decision Matrix
Axis 1 — Cost (CapEx + 5-year OpEx)
Switches, NICs, and cables combined. RoCEv2 400G is about 25-32% cheaper than IB NDR (based on Q1 2026 list-price comparison; negotiated prices vary).
Axis 2 — Ops Team RDMA Experience
- IB is mostly automatic with OpenSM, and diagnosed with ibdiagnet. - RoCEv2 requires PFC / ECN / DCQCN tuning, and RoCE counters / WQE drop analysis is hard. - For a team adopting it for the first time, RoCEv2 carries a 3-6 month learning curve.
Axis 3 — NCCL all-reduce Efficiency (workload-dependent)
- If small messages (e.g., MoE) are frequent, IB's sm-aware library + SHARP is advantageous. - If it's mostly large messages + synchronous training, RoCEv2 is plenty.
Decision Table
| Scenario | Recommendation | |---------|------| | LLM training (70B+), latency-sensitive | IB-NDR | | Inference/serving-focused, some fine-tuning | RoCEv2-400 | | Team adopting RDMA for the first time | IB (lower operational burden) | | Rich RDMA operations experience + TCO priority | RoCEv2 | | Multi-site (DCQCN consistency is hard) | IB |
Run a PoC
This decision should be made not by "what the vendor says" but by "the numbers measured on your workload." We recommend a 4-week PoC — two weeks each on both fabrics with an identical workload plus osu_allreduce.
/case-studies/roce-vs-infiniband-poc — a 28% TCO reduction case at a gaming subsidiary.