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At 128,000 accelerators, the limiting factor is no longer a switch specification. The cluster becomes a building-scale optical system in which rack layout, switching tiers, cable pathways, transceivers, timing, power, cooling and maintenance must be engineered together. AFL’s 2026 reference designs show a structural change from 16K systems: roughly eight times as many accelerators require about 24 times as much fiber.
The 128K design changes the unit of scale
In a smaller AI installation, a rack can be treated as a convenient packaging boundary. AFL’s 128K design treats the rack as the node. That change matters because the densest accelerator-to-accelerator mesh must be organized across many racks while keeping enough traffic local to avoid an unmanageable building-wide web of connections.
A third switching tier is used to keep the most demanding mesh inside each building. The result is not simply a larger version of a 16K fabric: it is a different physical and operational architecture.
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What the fiber counts look like
| Measure | 16K reference scale | 128K reference scale | What the figure means |
|---|---|---|---|
| Accelerator endpoints | 16,000 | 128,000 | The 128K design has eight times the endpoints. |
| Fiber in the scale-out fabric | Roughly 250,000 | About 2 million at the accelerator face | The 16K figure is for its scale-out fabric; the 128K figure is measured at the accelerator face. |
| Total fiber across switching tiers | Not stated | About 6.2 million across three tiers | This includes the three-tier 128K design. |
| Fiber growth versus accelerator growth | Baseline | About 24 times the fiber for about eight times the accelerators | AFL attributes the extra growth to twice the fibers per accelerator and 1.5 times the switching tiers. |
| Endpoints per network plane | Not stated | About 131,000 per plane | Figure from AFL’s reference-design discussion reported by Data Center Dynamics in 2026. |
Those counts describe physical termination, not just logical ports. Every additional trunk, cassette, patch panel and service loop consumes pathway space and creates another inspection and failure point. Alan Keizer, AFL’s senior technical adviser, summarized the practical problem: “Simply organizing the design, sourcing, supply, and installation of that many components is a challenge.”
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How one device fans out to 128,000 others
A synchronous training job needs a coherent, time-synchronous, non-blocking fabric. Each accelerator may participate in collective operations with many others, so a link failure or a congested path can stall work that is otherwise ready to run. The physical design therefore has to provide predictable paths, multiple network planes and enough switching capacity at every tier.
1. Keep short connections inside the rack
Use copper for the shortest internal reaches where its low power per bit, low cost and mature reliability are advantageous. NVIDIA’s Spectrum-X rack-scale architecture, for example, uses copper cable cartridges for short internal connections and front-panel optical connectivity for larger GPU domains. Its documentation describes scaling Ethernet to as many as 128K GPUs in two independent network tiers; that is a vendor architecture example, not the same topology as AFL’s three-tier reference design.
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2. Move between racks with optical links
Once a connection extends beyond a short rack reach, optical fiber supplies the tens-of-meters reach and the density needed for scale-out. Meta’s RoCE description uses single-mode fiber and 400G pluggable transceivers in a non-blocking back-end fabric. The transceiver speed, fiber mode, connector system and loss budget must be selected as one system rather than purchased independently.
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Independent planes can provide alternate paths and help isolate faults. A multi-tier design places switching functions where they reduce the longest or most congested paths. In AFL’s 128K design, the third tier helps keep the densest mesh within a building, limiting the amount of traffic that must cross a campus or inter-building boundary.
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Copper or fiber for an AI cluster?
| Design consideration | Copper | Optical fiber |
|---|---|---|
| Best reach | Typically less than 2 meters in the short-reach links discussed by Microsoft Research. | Tens of meters and longer scale-out paths. |
| Power and cooling | Efficient for short links; avoids optical conversion at those reaches. | Transceivers and optical electronics add power and cooling load. |
| Reliability evidence | Microsoft Research reported current optical links failing up to 100 times as often as comparable copper links in its 2025 analysis. | More components and connectors create more potential failure points, although fiber is essential for long reach and high-density scale-out. |
| Signal integrity | Reach and bandwidth are constrained as distance and speed rise. | Supports long, high-bandwidth runs but introduces transceiver, cleanliness and insertion-loss dependencies. |
| Recommended role | Short, controlled rack-internal connections. | Rack-to-rack, row-to-row and building-scale connectivity. |
The Microsoft Research failure comparison is a 2025 finding about current optical links, not a universal failure rate for every optical technology. It does, however, explain why a 128K deployment needs stricter inspection, telemetry, spares and fault isolation than a simple port-count exercise.
Where the building becomes the bottleneck
Pathways and termination fields
Millions of fibers must be routed with controlled bend radius, documented polarity and enough slack for service. Trunks, patch panels and cassettes can occupy more space than the active switches they connect. Pathway fill, separation of power and data, access for cleaning and the ability to replace one trunk without disturbing adjacent trunks should be fixed before equipment is ordered.
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Power and cooling at the optical edge
Optical modules at every tier convert electrical signals to light and back again. Their aggregate power becomes a cooling design input, especially when 400G or faster pluggables are used in large populations. Rack-level copper can reduce conversion power for short links, while optical scale-out avoids the reach and signal-integrity limits of copper. The efficient design uses each medium where its physical characteristics fit.
Synchronization and congestion
Training collectives are sensitive to stalls. A fabric that is technically connected but blocking under load can waste the entire job. Time synchronization, non-blocking switch capacity, balanced planes and monitoring for hot spots are therefore architectural requirements, not tuning tasks left for deployment.
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Protection and recovery have to be designed in
With millions of terminations, failures are inevitable even when individual components are reliable. Use independent planes where possible, document which paths share a trunk or patch field, and make fault domains visible to the scheduler and network controller. ITU-T work on optical circuit switching and hitless protection switching illustrates the direction: reroute or switch around a failed optical path without forcing a synchronized workload to restart whenever the technology and control plane support it.
- Inspect and clean every connector before insertion; contamination can consume the insertion-loss margin.
- Record polarity, fiber identity, transceiver type and patch-panel location in the installation database.
- Keep tested spare transceivers, trunks and patch components matched to each link class.
- Monitor optical power, error counters and link flaps per plane so a degrading path is found before a training job fails.
- Test failover with production-like collective traffic rather than relying only on link-up status.
What to specify before buying fiber trunks
The directly relevant physical component for a dense deployment is an MPO/MTP single-mode fiber-optic trunk cable, but “fiber cable” is not a sufficient purchase specification. Confirm all of the following with the network and cabling engineers:
- Connector gender and polarity: define the end-to-end method, including whether the installation uses Type A, Type B or another documented polarity scheme.
- Fiber count: match the trunk to the cassette, patch panel and transceiver breakout plan.
- Insertion-loss budget: include every connector, splice, cassette and planned service connection.
- Reach and bend radius: verify the actual pathway route, slack storage and manufacturer limits.
- Patch-panel compatibility: ensure the trunk, cassettes and adapters use the same connector family and polarity documentation.
- Transceiver standard: select for the intended 400G, 800G or other optical interface, fiber mode and breakout arrangement.
- Inspection and acceptance: require end-face inspection, cleaning records and optical-loss test results before the cable enters the live fabric.
Can the design grow beyond one AI zone?
A 128K zone should be treated as a repeatable building block, not as an isolated maximum. Reserve pathway capacity, switch ports, power and cooling for the next zone, and define which traffic is allowed to cross the building boundary. The key question is whether expansion adds another self-contained non-blocking plane or forces existing collectives onto longer, shared paths.
AFL’s numbers make the trade-off clear: eight times the accelerators can require about 24 times the fiber. At this scale, architecture, sourcing and installation are inseparable from networking performance. The practical answer is a layered fabric—copper for short rack reaches, optical links for scale-out, multiple planes or tiers for predictable paths, and disciplined protection and cabling processes for everything in between.
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