51.2Tbps 800G Spine-Leaf Architecture for Large-Scale 8,192-GPU AI Clusters

Large-scale AI training is changing the way data center networks are designed.

As GPU clusters expand from dozens of accelerators to thousands, the network must move enormous amounts of data between compute nodes without becoming a bottleneck.

For a large AI cluster based on switches with 51.2Tbps unidirectional switching capacity, a two-layer Spine-Leaf architecture can provide the bandwidth density and scalability required for modern distributed AI workloads.

With a 1:1 convergence ratio, this architecture can support up to:

8,192 GPUs

across:

1,024 AI servers or compute nodes

with each server configured with:

8 NVIDIA H100 GPUs

and

8 × 400G NICs

The result is a high-bandwidth fabric designed for large-scale training, inference, distributed storage, and GPU-to-GPU communication.

For Taiwan manufacturer Optech, this architecture also creates demand for a complete portfolio of 800G and 400G physical-layer connectivity, including OSFP, QSFP-DD, QSFP112, breakout optics, DAC, ACC, AOC, and fiber solutions.

Why 51.2Tbps Switches Matter for AI Networks

A switch with 51.2Tbps one-way switching capacity provides enough internal bandwidth to support a large number of high-speed ports.

Common configurations can include:

• 64 × 800GbE
• 64 × 800GbE QSFP-DD
• 128 × 400GbE QSFP112

This flexibility allows network architects to select the switch form factor that best matches their server, NIC, cabling, and deployment strategy.

The architecture described here is based on the widely adopted:

64-port 800GbE OSFP switch

This provides a strong foundation for high-density Spine and Leaf layers while also allowing 800G ports to be divided into 400G server-facing connections.

What Does a 1:1 Convergence Ratio Mean?

In an AI fabric, bandwidth between the server-facing side and the network fabric is critical.

A 1:1 convergence ratio means the architecture is designed so that the available uplink capacity closely matches the server-facing bandwidth.

In practical terms:

400G NIC traffic entering the Leaf layer

is matched by

sufficient uplink bandwidth toward the Spine layer

rather than being heavily oversubscribed.

This is important for AI because distributed workloads can generate large all-to-all and east-west traffic patterns.

A 1:1 design helps reduce the risk that the network fabric becomes the limiting factor during:

• distributed training
• gradient exchange
• collective communication
• model synchronization
• distributed inference
• checkpointing
• storage access

Supporting 1,024 Servers and 8,192 GPUs

The cluster design includes:

1,024 servers

with:

8 GPUs per server

giving:

1,024 × 8 = 8,192 GPUs

Each server also includes:

8 × 400G NICs

which means the entire fabric can contain as many as:

8,192 high-speed 400G NIC connections

This illustrates the scale of modern AI networking.

At this level, even small differences in transceiver cost, power consumption, cable length, or compatibility can have a major impact on the total project.

Each Server: 8 NVIDIA H100 GPUs + 8 × 400G NICs

The server architecture described here uses eight NVIDIA H100 GPUs per node.

Each server also includes eight 400G network interfaces, allowing high-bandwidth communication between compute nodes and the Leaf switching layer.

A simplified server connection model is:

8 × NVIDIA H100 GPUs

↓

8 × 400G NICs

↓

Leaf Switch

↓

800G Spine-Leaf Fabric

This type of design provides multiple independent high-speed network paths for each server.

Flexible NIC Options

One of the strengths of this architecture is that customers are not locked into only one NIC form factor.

Two important server-side options include:

NVIDIA ConnectX-7 with OSFP

ConnectX-7-based 400G networking can use an OSFP interface, making it suitable for environments already standardized around OSFP connectivity.

Typical physical-layer options can include:

400G OSFP SR4

or

400G OSFP DR4

depending on distance.

NVIDIA BlueField-3 DPU / SuperNIC with QSFP112

Another option is a 400G QSFP112 interface, such as those used in certain BlueField-3 DPU or SuperNIC architectures.

This gives network architects a different mechanical and cabling option while maintaining 400G server-side connectivity.

For Optech, this means supporting both:

800G OSFP → 400G OSFP

and

800G OSFP → 400G QSFP112

network architectures.

Flexible Switch Options

The same network concept can also support different switch form factors.

The main options described include:

This flexibility is valuable because different data center operators may standardize on different physical interfaces.

The architecture used in this design is based on the 64-port 800GbE OSFP switch, but the underlying Spine-Leaf concept can be adapted to other switch configurations.

Why 64-Port 800G OSFP Is Attractive

A 64-port 800G switch provides very high port density.

One 800G port can also support two 400G logical connections when the host platform and optical architecture support breakout.

This means:

1 × 800G Port

can effectively serve:

2 × 400G NIC Links

This is especially useful when server-side NICs operate at 400G.

800G-to-400G Breakout: A Key AI Fabric Design

One of the most important characteristics of this architecture is the combination of:

800G switching

with

400G server-side networking

Rather than requiring every endpoint to operate at 800G, the network can use high-density 800G ports on the switch side and divide them into two 400G server connections.

This provides several benefits:

• higher switch port utilization
• lower switch count
• flexible 400G endpoint support
• easier migration toward 800G
• better compatibility with current-generation NICs
• reduced rack-space requirements

Optech 800G OSFP 2xSR4 Solutions

For short-reach multimode applications, an 800G OSFP 2xSR4 architecture can provide two 400G optical links from one 800G switch interface.

For example:

800G OSFP 2xSR4

can connect toward:

2 × 400G OSFP SR4

or

2 × 400G QSFP112 SR4

depending on the server-side interface.

This is particularly useful for shorter Leaf-to-Server connections.

Optech 800G OSFP 2xDR4 Solutions

For medium-reach single-mode connectivity, 800G OSFP 2xDR4 provides another important option.

One 800G OSFP port can provide:

2 × 400G DR4

allowing connection to:

• 400G OSFP DR4 NICs
• 400G QSFP112 DR4 NICs or DPUs

This type of architecture can be particularly attractive for large AI halls where switch-to-server distance exceeds practical copper or short-reach multimode limits.

800G DAC / ACC for Very Short AI Links

Not every AI network link requires fiber.

For connections of only a few meters, DAC or ACC can provide a more efficient alternative.

DAC

Passive Direct Attach Copper is attractive because of:

• low power
• low latency
• lower cost
• simple deployment

ACC

Active Copper Cable can provide signal conditioning while maintaining a copper-based architecture.

These products are particularly useful for:

• same-rack connections
• adjacent racks
• short server-to-switch links
• short switch-to-switch links

Choosing Connectivity by Distance

A cost-efficient AI network should use the right technology for each physical distance.

A typical approach may look like:

This avoids using expensive long-reach optical modules where simpler technologies would be sufficient.

Why Form-Factor Flexibility Matters

AI data centers are rarely built around one single connector type.

The switch may use:

OSFP

while the NIC uses:

OSFP

or:

QSFP112

Another customer may select QSFP-DD switching.

This creates a need for connectivity suppliers that can support mixed-form-factor environments.

Optech can help customers design connectivity around:

• OSFP
• QSFP-DD
• QSFP112
• 400G
• 800G
• breakout architectures

Optech Connectivity for the Physical AI Fabric

For this type of large AI network, Optech can provide physical-layer products including:

• 800G OSFP
• 800G 2xSR4
• 800G 2xDR4
• 800G 2xFR4
• 400G OSFP SR4
• 400G OSFP DR4
• 400G QSFP112 SR4
• 400G QSFP112 DR4
• 800G DAC
• ACC
• AOC
• breakout cable solutions

This allows customers to source multiple network layers from one supplier.

From Architecture to Optical BOM

At the scale of 8,192 GPUs and 8,192 NICs, network planning must eventually become a detailed Bill of Materials.

For every link, the customer needs to identify:

Port A

Port B

Speed

Form Factor

Distance

Optical or Copper Technology

Fiber / Cable

Quantity

A practical BOM might contain categories such as:

Spine-to-Leaf

800G OSFP ↔ 800G OSFP

using:

• 2xSR4
• 2xDR4
• 2xFR4
• DAC / ACC

Leaf-to-Server with ConnectX-7

800G OSFP →

400G OSFP

using:

• SR4
• DR4
• DAC where appropriate

Leaf-to-Server with BlueField-3 / QSFP112

800G OSFP →

400G QSFP112

using:

• SR4
• DR4
• breakout DAC

Optech can work with customers to translate a topology into a practical optical and cable BOM.

Why BOM Optimization Matters at 8,192-GPU Scale

In a small network, a difference of a few dollars or a few watts per link may not seem important.

At thousands of ports, however, these differences become significant.

For example, product selection affects:

• total CAPEX
• power consumption
• cooling requirements
• cable management
• spare inventory
• installation labor
• maintenance complexity

This is why selecting the correct interconnect technology at each layer matters.

Optech Customization Capability

Large AI deployments often require project-specific customization.

Optech can work with customers on:

Cable Length

DAC, ACC and AOC can be optimized according to rack placement.

Platform Compatibility

Products can be configured according to the target switch, NIC, or DPU.

EEPROM Coding

Module coding can be adjusted where required for host compatibility.

Breakout Architecture

Optech can evaluate 800G-to-2x400G optical and copper breakout requirements.

Labels and Serial Numbers

Large AI projects may require:

• customer P/N
• serial-number control
• rack identification
• QR codes
• barcodes
• project labels

BOM Planning

Optech can help organize the required module and cable quantities by network layer and distance.

Typical Applications

This architecture is particularly relevant to:

Large-Scale AI Training

Thousands of GPUs require high-bandwidth communication during distributed model training.

Generative AI

Large language models generate intensive GPU-to-GPU communication across the cluster.

AI Inference

Distributed inference can require high-throughput access to compute and storage infrastructure.

HPC

High-performance computing benefits from a high-bandwidth, low-oversubscription fabric.

Cloud AI

Cloud providers need network architectures that can grow as GPU resources increase.

Distributed Storage

AI infrastructure requires fast movement of training datasets and checkpoints.

Why Choose Optech for Large AI Fabrics?

Optech can support customers with more than individual optical modules.

The company can provide:

400G + 800G connectivity

OSFP + QSFP112 + QSFP-DD options

Optical + DAC + ACC + AOC

800G-to-400G Breakout

Custom Cable Lengths

Platform Compatibility

EEPROM Coding

Topology-to-BOM Support

Sample Qualification

Volume Production

For large AI projects, this makes it easier to manage the physical connectivity layer through one supplier.

Ready to Scale from Hundreds to Thousands of GPUs?

The transition toward 8,000+ GPU AI clusters requires a different network design philosophy.

A high-performance fabric must combine:

51.2Tbps switches

800G Spine-Leaf connectivity

1:1 convergence

400G server-side NICs

and

flexible OSFP / QSFP-DD / QSFP112 form factors

The architecture described here demonstrates how a two-layer network can support up to:

1,024 servers

and:

8,192 NVIDIA H100 GPUs

while maintaining flexibility across switch and NIC platforms.

With a broad portfolio of 800G and 400G optical transceivers, DAC, ACC, AOC and custom breakout solutions, Optech can help customers turn large-scale AI network designs into deployable physical connectivity solutions.

Contact Optech for architecture review, product recommendations, BOM planning, samples, project pricing, and volume orders.

 

FAQ

1. How many GPUs can this AI architecture support?

The architecture described supports up to 8,192 GPUs across 1,024 servers, assuming eight GPUs per server.

2. How many NICs are used?

Each server is configured with eight 400G NICs, giving up to 8,192 400G NIC connections across 1,024 servers.

3. What is the switch capacity?

The design is based on switches with 51.2Tbps unidirectional switching capacity.

4. What does a 1:1 convergence ratio mean?

It means the network is designed so the fabric-facing bandwidth is matched closely to the server-facing capacity, reducing oversubscription and supporting demanding east-west AI traffic.

5. Which switch interfaces can be used?

The described options include:

• 64-port 800GbE OSFP
• 64-port 800GbE QSFP-DD
• 128-port 400GbE QSFP112

The reference architecture discussed here uses 64-port 800GbE OSFP switches.

6. Which NIC options are supported?

The architecture can support different 400G NIC form factors, including examples such as:

• NVIDIA ConnectX-7 with OSFP
• BlueField-3 DPU / SuperNIC with QSFP112

7. Why use 800G switches with 400G NICs?

One 800G switch port can be divided into two 400G links in suitable breakout architectures, improving port density and allowing current-generation 400G endpoints to connect to an 800G fabric.

8. What optics can be used for 800G-to-400G breakout?

Depending on distance, examples include:

• 800G OSFP 2xSR4 → 400G SR4
• 800G OSFP 2xDR4 → 400G DR4
• short-reach breakout DAC solutions

9. Can the server side use both OSFP and QSFP112?

Yes. This architecture is designed to provide flexibility for both 400G OSFP NICs and 400G QSFP112 DPU / SuperNIC interfaces.

10. Can Optech help convert the topology into a BOM?

Yes. Customers can provide their switch quantity, server quantity, NIC type, distances, rack layout, and preferred connection type, and Optech can help map the topology to appropriate optical modules, DAC, ACC, AOC and breakout products.

Conclusion

Building an AI cluster with 8,192 GPUs requires more than simply adding more servers.

The network must scale at the same rate as the compute infrastructure.

A two-layer 800G Spine-Leaf fabric using 51.2Tbps switches and a 1:1 convergence ratio provides a practical architecture for connecting up to 1,024 servers equipped with eight GPUs and eight 400G NICs each.

Equally important, the architecture remains flexible.

Customers can choose among:

800G OSFP

800G QSFP-DD

400G QSFP112

400G OSFP

and different optical or copper technologies depending on the server, switch and distance.

For Optech, this creates a complete physical-layer opportunity: from 800G Spine-Leaf optics to 400G NIC connectivity, breakout modules, DAC, ACC and AOC, Optech can support customers from architecture planning through BOM creation and volume deployment.

Optech welcomes large-scale AI cluster projects, topology reviews, sample qualification, customized connectivity requirements, and volume orders.