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Freedom251 › Blog › AI networking

How to Tell Whether Your Network Can Handle AI’s East-West Traffic

By Freedom251 Team · · 8 min read

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Your network is ready for AI only if it can move synchronized, high-volume traffic between accelerators without letting a few congested flows hold up an entire training operation. A high average link-utilization figure—or a fast port by itself—cannot establish that. Assess the whole fabric, from NICs and cables to topology, congestion control, telemetry, software integration, and failure recovery, using benchmarks that resemble the jobs you intend to run.

Why do AI workloads put different demands on a network?

Distributed AI training generates large amounts of east-west traffic: data moving among GPUs and hosts inside a cluster, rather than mainly between a data center and outside users. Operations such as AllReduce, AllGather, and AlltoAll coordinate data exchange across participants. Their traffic can arrive in correlated bursts, unlike the many comparatively independent flows typical of conventional data-center workloads.

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That synchronization changes the impact of congestion. A collective operation may not finish until its slowest flow completes. If one large flow is delayed, many participating GPUs can wait, even when other links have spare capacity. The cost of a network bottleneck is therefore not limited to a slow transfer: it can extend job completion time and leave expensive accelerators idle.

Why a straggler can matter more than the average

Consider a collective in which traffic takes several paths through the fabric. If one path becomes congested, its delayed flow can determine when the operation completes. A report showing modest average utilization across the cluster could miss that short-lived bottleneck. Queue buildup, incast, hash collisions, or tail latency can still affect completion time.

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Static ECMP (Equal-Cost Multi-Path) hashing is one potential source of imbalance. It assigns flows to available paths using a hash; large flows that collide on the same link can overload it while other links remain underused. If congestion fills buffers, packets may be lost and retransmitted, adding further delay. The important question is not simply how much traffic the network carries on average, but how it behaves when large synchronized flows compete for paths.

How can I tell if my network is fast enough for AI?

There is no single port speed or average-utilization threshold in the available architecture examples that certifies a fabric as AI-ready. Measure the traffic patterns and outcomes of your actual workloads, then check whether the entire path can carry them at the scale you need.

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Start by identifying the training and inference patterns the network must support. Measure AllReduce, AllGather, and AlltoAll traffic for training, along with the inference request patterns relevant to your service. Then map each host-to-leaf, leaf-to-spine, and inter-zone path, and document the oversubscription ratio at every point. An oversubscribed link is one whose potential downstream demand exceeds its upstream capacity; knowing where that happens makes it possible to assess whether the resulting contention is acceptable for the target jobs.

Use job-completion time and accelerator utilization to judge whether the fabric is helping the application finish efficiently. Collective throughput, tail latency, congestion behavior, and loss also help explain a poor result. Compare measurements at realistic scale and during soak tests: a small test or a short peak-rate measurement does not show how the network behaves under sustained, distributed load.

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Questions to answer before calling a fabric ready

  • Which collective operations and inference request patterns do the intended workloads generate?
  • What are the aggregate and bisection bandwidths, and where does oversubscription occur?
  • How do congestion controls and queue behavior affect latency, loss, and pause propagation?
  • Can operators see congestion at the port and queue level, and receive actionable alerts?
  • Have the NICs, switches, optics, cables, host lanes, drivers, and software stack been validated together?
  • Does a realistic-scale test show acceptable job-completion time and accelerator utilization, including during a soak test or after a failure?

Do AI clusters need 400GbE or 800GbE?

Both 400Gb/s and 800Gb/s appear in NVIDIA’s current AI architecture references, but those are component link capabilities, not universal per-GPU requirements or a rule for choosing a fabric. The right design depends on workload demand, topology, oversubscription, and the behavior of the full system.

Reference component Documented capability What the figure establishes
BlueField-3 SuperNIC Up to 400Gb/s NVIDIA’s reference architecture describes low-latency connectivity, RDMA/RoCE acceleration, and GPUDirect features.
ConnectX-8 SuperNIC Up to 800Gb/s NVIDIA’s NVL72 architecture specifies this capability for GPU-to-GPU traffic and describes congestion control, telemetry-based routing, and QoS capabilities.

“Up to” matters: the listed rate is a maximum specified for the named SuperNIC in the referenced architecture, not a guarantee of application throughput. A system-level decision must also account for the switch ports and topology, optics or direct-attach cables, host connectivity, and congestion management. A fast adapter cannot compensate for an undersized or poorly balanced path elsewhere in the fabric.

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Use the workload and topology measurements to decide whether a particular link rate makes sense. Compare aggregate and bisection bandwidth, collective throughput, tail latency, power and port density, cabling reach, software support, recovery behavior, and total cost per GPU or host. The architecture examples establish that 400Gb/s and 800Gb/s are real deployment points; they do not establish a blanket rule that every AI cluster needs either one.

How should an AI network fabric be designed?

Design the fabric end to end: NICs, switches, topology, optics and cabling, congestion controls, telemetry, software integration, and recovery all affect the result. Treating any one component’s nominal bandwidth as a proxy for system performance can hide a bottleneck elsewhere.

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Topology and oversubscription

Map capacity through the complete route, including host-to-leaf, leaf-to-spine, and any links between zones. Calculate aggregate and bisection bandwidth, and write down each oversubscription ratio rather than describing the network only by its fastest link. This reveals where the fabric intentionally concentrates traffic and helps focus tests on those paths.

Meta’s 2024 account of its distributed AI network describes an AI Zone built as a two-stage Clos. Rack training switches provide scale-up connectivity; cluster training switches provide scale-out connectivity; and 400G pluggable transceivers connect the tiers. Cross-zone connectivity is intentionally oversubscribed and managed with ECMP plus workload-aware scheduling. This is an example of a design for a particular large environment, not a universal topology prescription.

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Congestion management and visibility

Decide how the fabric will handle congestion, and verify the behavior rather than relying on feature lists. The IETF AI-fabric benchmarking methodology covers RoCEv2 and UET, PFC, ECN, DCQCN, credit-based flow control, ECMP, dynamic load balancing, and packet spraying. The inclusion of multiple mechanisms is a reason to test the chosen combination in context, not to assume that enabling one feature resolves every source of congestion.

Require per-port and per-queue telemetry, visibility into congestion, and alerts that operators can act on. During tests, observe queue depth, loss, pause propagation, and tail latency alongside throughput. Without that visibility, a weak job result may show that something is wrong without revealing whether the issue is a congested path, an imbalanced flow distribution, or another part of the system.

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Compatibility across the full path

Match the NIC, switch, optics, direct-attach or active optical cables, PCIe lanes, and cable reach to one another. Validate the driver, RDMA/RoCE stack, collective library, orchestration, and failure-reroute integration as a system. A component that supports a feature on its own may not deliver the intended behavior when combined with the rest of the production software and hardware stack.

How should I benchmark an AI fabric before deployment?

Use application-shaped traffic and evaluate application outcomes, not just a link-rate demonstration. The IETF’s “Benchmarking Methodology for AI Training Network Fabrics” lays out a test plan covering transports, congestion-control approaches, load balancing, collective operations, and scale/soak tests.

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  1. Reproduce the workload mix. Run AllReduce, AllGather, and AlltoAll patterns, and include relevant inference request patterns. Avoid treating a generic traffic stream as a substitute for the workload being deployed.
  2. Record application-level results. Measure job-completion time, training throughput, and accelerator utilization. These show whether network behavior is affecting work the organization cares about.
  3. Measure network behavior that explains the result. Track collective throughput, tail latency, congestion, loss, queue behavior, and the effects of the selected flow-control and load-balancing approaches.
  4. Test at the intended scale. Exercise the topology and oversubscribed paths that production will use. Include scale and soak runs so the evaluation covers sustained operation, not only a brief run.
  5. Exercise failure and recovery. Check that rerouting and the surrounding software stack behave as intended when a component or path is unavailable. Record the impact on job completion and accelerator utilization.
  6. Compare candidate designs on the same basis. Use the same workloads and outcome metrics when comparing fabrics, and include power, port density, optics and cabling, software ecosystem, and total cost per GPU or host.

The benchmark methodology is an Internet-Draft, so treat it as a practical framework rather than a guarantee that every deployment has a single mandatory test recipe. Its value is in making tests more representative and comparable, including tests of AllReduce, AlltoAll, AllGather, ECMP, dynamic load balancing, packet spraying, and the listed congestion-control options.

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What should I check when buying or upgrading networking?

Start from a measured bottleneck and a target workload, not from a headline port speed. The design checklist below helps turn that evidence into a purchase or upgrade decision.

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  • Capacity: Record aggregate and bisection bandwidth, path-level capacity, and every oversubscription ratio.
  • Workload performance: Compare collective throughput, job-completion time, training throughput, accelerator utilization, and tail latency.
  • Congestion response: Evaluate loss handling, queue depth, pause propagation, and the chosen ECN/PFC/DCQCN or UET behavior.
  • Path balancing: Assess ECMP and any dynamic load balancing or packet-spraying approach under large, correlated flows.
  • Observability: Confirm that operators can inspect per-port and per-queue conditions and act on congestion alerts.
  • Physical fit: Match link rates and reach across NICs, switches, optics, DAC/AOC cabling, and available PCIe lanes.
  • Operational fit: Test drivers, RDMA/RoCE, collective libraries, orchestration, and failure rerouting together.
  • Economics: Compare power, port density, software ecosystem, recovery behavior, and total cost per GPU or host alongside bandwidth.

If an existing fabric performs poorly, use telemetry and workload-shaped tests to locate the constraint before replacing equipment. The limiting factor may be an oversubscribed tier, flow imbalance, queueing, an incompatible software path, or a failure-recovery issue; upgrading one link without identifying the cause may leave the job-level problem intact.

What does an AI-ready network decision come down to?

The practical decision is whether the fabric sustains the workload’s synchronized exchanges at the needed scale without stragglers, congestion, or poor recovery undermining job performance. NVIDIA’s architecture references establish 400Gb/s and 800Gb/s SuperNIC examples; Meta’s published topology shows how a large AI zone can combine scale-up, scale-out, and managed cross-zone oversubscription; and the IETF draft provides a benchmark framework. None replaces workload-specific validation of the system you plan to operate.

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Frequently Asked Questions

Does an AI cluster need 400Gb/s or 800Gb/s per GPU?

The cited architecture references specify maximum SuperNIC link capabilities, not a minimum bandwidth allocation for every GPU. The sources do not establish a universal per-GPU target; derive one from workload measurements and the capacity of the complete path.

Can I judge AI network performance from average port utilization?

No. A low average can conceal short periods of queue buildup, flow collisions, or high tail latency that delay a collective. Look at workload completion and congestion telemetry as well as averages.

Is an Internet-Draft the same as a finalized networking standard?

No. The cited AI-fabric benchmarking methodology is identified as an Internet-Draft, so it should be treated as a published test framework rather than a finalized standard. Its scenarios can still help make evaluations more systematic.

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