400G Spine-Leaf Network Solutions for AI, HPC and High-Density Data Centers

As AI, cloud computing, high-performance computing, and large-scale data centers continue to expand, network architecture must support more endpoints, higher bandwidth, and faster east-west traffic.

A traditional hierarchical network can become difficult to scale when thousands of servers, NICs, and accelerators need to communicate simultaneously.

For this reason, Spine-Leaf architecture has become an important network design for modern high-performance data centers.

The architecture shown in the diagram illustrates a scalable 400G environment connecting multiple server groups, Leaf switches, Spine switches, and as many as 2,048 NICs across eight groups.

For Taiwan manufacturer Optech, this type of network can be supported with a complete portfolio of:

• 400G QSFP-DD optical transceivers
• 400G OSFP optical transceivers
• SR8
• SR4
• DR4
• FR4
• DAC
• AOC
• customized high-speed connectivity solutions

By selecting the right optical architecture for each link distance, customers can optimize performance, cabling complexity, and overall deployment cost.

What Is a Spine-Leaf Network Architecture?

A Spine-Leaf network uses two primary switching layers:

Spine Layer

and

Leaf Layer

Each Leaf switch connects to multiple Spine switches, creating multiple parallel paths through the network.

Servers, NICs, storage systems, and compute nodes connect to the Leaf layer.

The basic architecture can be represented as:

Servers / NICs → Leaf Switches → Spine Switches

Unlike traditional three-tier networking, Spine-Leaf is optimized for large volumes of east-west traffic.

This is especially important in:

• AI training clusters
• GPU networks
• HPC systems
• cloud infrastructure
• distributed storage
• large-scale data centers

Why 400G Is Important for Spine-Leaf Networks

Modern AI and HPC environments generate massive amounts of traffic between servers.

GPU training workloads, distributed storage, and large datasets often require data to move continuously between compute nodes rather than simply from server to user.

400G connectivity helps increase network bandwidth density across both:

Leaf-to-Server links

and

Spine-to-Leaf links

This allows data centers to scale while reducing the number of lower-speed physical connections required.

Optech 400G Connectivity for Spine-to-Leaf Links

The diagram highlights several connectivity options between Spine and Leaf switches.

The correct solution depends primarily on:

• link distance
• fiber infrastructure
• connector type
• switch interface
• power requirements
• cabling density

400G QSFP-DD SR8 for Up to 100m

For relatively short Spine-to-Leaf connections, the diagram shows:

400G QSFP-DD SR8 ↔ 400G QSFP-DD SR8

with a reach of up to approximately:

100 meters

SR8 is designed for high-speed short-reach multimode fiber applications.

This can be useful when Spine and Leaf switches are located within the same data hall or nearby network zones.

Typical applications include:

• intra-data-center switch links
• high-density Leaf-Spine fabrics
• short-distance AI networking
• multimode structured cabling

400G QSFP-DD DR4 for Up to 500m

For longer Spine-to-Leaf connections, the architecture shows:

400G QSFP-DD DR4 ↔ 400G QSFP-DD DR4

with a reach of up to:

500 meters

DR4 uses parallel single-mode fiber and is widely used in modern high-speed data center environments.

Compared with shorter-reach multimode solutions, DR4 provides additional deployment flexibility for larger facilities.

Typical applications include:

• large data halls
• Spine-to-Leaf connectivity
• AI cluster fabrics
• cloud data centers
• high-density switch interconnects

400G QSFP-DD FR4 for Up to 2km

When network distance extends beyond typical data center hall dimensions, the diagram shows:

400G QSFP-DD FR4 ↔ 400G QSFP-DD FR4

with reach up to:

2 kilometers

FR4 uses wavelength-division multiplexing and duplex single-mode fiber.

This makes it particularly useful when customers need:

• longer reach
• duplex LC infrastructure
• lower fiber count
• connections between network rooms
• campus-style data center connectivity

For large AI campuses or distributed data center facilities, FR4 can provide a practical long-distance 400G option.

400G DAC / AOC for Short-Distance Connections

For very short Spine-to-Leaf connections, optical transceivers are not always necessary.

The diagram also includes:

400G QSFP-DD DAC / AOC

for approximately:

0.5m to 30m

The exact technology can be selected according to distance.

400G DAC

DAC is particularly attractive for short connections because it can provide:

• low power consumption
• low latency
• simple installation
• cost-effective deployment

Typical applications include same-rack or adjacent-rack links.

400G AOC

AOC can provide greater distance and easier cable routing compared with passive copper.

It is useful when customers need:

• lighter cabling
• longer integrated links
• simple plug-and-play deployment
• high-density rack connectivity

Leaf-to-Server Connectivity Requires Different Design Choices

The Leaf-to-Server section of the diagram highlights an important aspect of AI data center design.

The switch side may use QSFP-DD, while the server or NIC side may use OSFP.

This means a network may require mixed-form-factor connectivity.

Optech can provide several options for these applications.

400G QSFP-DD SR8 to 400G OSFP SR8

One solution shown in the diagram is:

400G QSFP-DD SR8 → 400G OSFP SR8

with a reach of approximately:

50 meters

This type of connection is suitable for short-reach multimode fiber links between Leaf switches and server-side networking equipment.

Potential applications include:

• GPU servers
• AI compute nodes
• HPC servers
• high-performance NICs

400G QSFP-DD SR4 to 400G OSFP SR4

Another short-reach option shown is:

400G QSFP-DD SR4 → 400G OSFP SR4

with a reach of approximately:

50 meters

The diagram also highlights SR8 as an alternative for short-distance server connections.

For customers evaluating SR4 versus SR8, factors may include:

• port architecture
• fiber configuration
• cost
• module availability
• platform compatibility
• topology design

The product choice should be validated against the specific switch and NIC environment.

400G QSFP-DD DR4 to 400G OSFP DR4

For longer Leaf-to-Server connections, the diagram shows:

400G QSFP-DD DR4 → 400G OSFP DR4

with a reach of up to:

500 meters

This gives network architects additional flexibility when GPU servers, storage systems, or compute equipment are located farther from the Leaf switches.

400G SR8 vs SR4 vs DR4 vs FR4

Choosing the correct 400G optical module depends on the physical network design.

Instead of using one optical technology for every link, a well-designed data center can use different products according to actual distance.

Building a Cost-Optimized 400G Network

One of the most common mistakes in data center design is using a longer-reach optical module than the application actually requires.

For example, a 5-meter connection does not normally need a 2km optical transceiver.

A more efficient approach is to select:

DAC for the shortest links

AOC for short integrated optical connections

SR-class optics for short fiber links

DR4 for medium-distance data center links

FR4 for longer single-mode connections

This can help optimize both capital cost and network complexity.

400G Connectivity for Large AI Clusters

The architecture shown in the image illustrates a network scaling across multiple groups and thousands of NICs.

At this scale, optical planning becomes an important part of AI cluster design.

Network architects need to consider:

• number of Spine switches
• number of Leaf switches
• number of NICs
• oversubscription ratio
• physical rack placement
• cable lengths
• fiber type
• connector type
• transceiver quantity
• spare inventory

Optech can work with customers during this planning stage to recommend appropriate optical and cable products.

Why Mixed QSFP-DD and OSFP Networks Matter

Modern AI infrastructure often uses different form factors at different points in the network.

For example:

Switch Side: QSFP-DD

while

NIC / Server Side: OSFP

This creates demand for interoperable optical solutions capable of linking different physical form factors while maintaining the same 400G Ethernet connection.

Optech can support mixed-form-factor designs using:

• QSFP-DD optical transceivers
• OSFP optical transceivers
• compatible fiber connections
• DAC / AOC
• custom cable assemblies

Optech Customization Capability

Large-scale AI and data center networks rarely use only standard catalog configurations.

Different customers may require different:

• distances
• form factors
• connectors
• fiber types
• cable lengths
• EEPROM coding
• platform compatibility
• labels
• serial numbers

Optech can support project-specific customization.

Optical Reach

Customers can select products based on the actual link distance rather than purchasing one module for every network zone.

QSFP-DD and OSFP Compatibility

Optech can work with customers on mixed-form-factor connectivity between switches and server-side interfaces.

Custom DAC / AOC Length

For short-reach links, Optech can support project-specific cable lengths to improve rack organization.

Platform Coding

Modules can be configured according to target switch or NIC requirements where applicable.

Customized Labels and Serial Numbers

For AI projects involving thousands of modules, customers may require:

• project P/N
• serial-number management
• customer labels
• QR codes
• barcodes
• rack or deployment identification

Optech can support these requirements.

From Network Diagram to BOM

One of the most useful ways to design a large 400G AI network is to convert the topology directly into a Bill of Materials.

For each network layer, customers can define:

Link Type → Quantity → Distance → Form Factor → Optical Type → Fiber / Cable

For example:

Spine-to-Leaf
400G QSFP-DD DR4
500m-class connectivity

Leaf-to-Server
400G QSFP-DD DR4 → 400G OSFP DR4

Short Rack Links
400G DAC / AOC

Optech can help customers transform a network topology into an optical transceiver and cable BOM for project evaluation.

Typical Application Scenarios

AI Training Clusters

Large GPU clusters generate massive east-west traffic and require high-bandwidth communication between compute nodes.

400G Spine-Leaf connectivity provides the bandwidth needed to scale these environments.

High-Performance Computing

HPC networks require predictable, high-speed connections among compute, storage, and switching infrastructure.

Cloud Data Centers

Cloud infrastructure benefits from Spine-Leaf architectures because capacity can be expanded by adding additional Leaf and Spine switches.

Storage Networks

Distributed storage systems generate significant east-west traffic and can benefit from high-capacity 400G links.

Enterprise Data Centers

Enterprises upgrading from 100G toward 400G can use Spine-Leaf architecture to improve scalability and prepare for future growth.

Why Choose Optech for 400G Spine-Leaf Connectivity?

Optech can support customers with a broad 400G connectivity portfolio including:

QSFP-DD SR8

QSFP-DD SR4

QSFP-DD DR4

QSFP-DD FR4

OSFP SR8

OSFP SR4

OSFP DR4

DAC

AOC

and other high-speed optical and cable solutions.

As a Taiwan manufacturer, Optech can also support:

• project customization
• compatibility coding
• mixed-form-factor connectivity
• custom cable lengths
• optical module qualification
• labeling and serial numbers
• sample testing
• volume production

Ready to Build Your 400G AI Network?

A scalable AI network requires more than selecting a 400G switch.

The optical interconnect must be designed around:

distance + topology + form factor + fiber infrastructure + cost

The architecture shown in the diagram demonstrates how multiple connectivity technologies can work together:

SR8 / SR4 for short reach

DR4 for medium reach

FR4 for longer reach

DAC / AOC for very short high-density links

QSFP-DD and OSFP for mixed switch and server environments

Optech can help customers select the appropriate 400G optics and high-speed cables for each part of the network.

Contact Optech for topology review, product recommendations, sample qualification, BOM planning, project pricing, and volume orders.

 

FAQ

1. What is a 400G Spine-Leaf network?

A 400G Spine-Leaf network uses high-speed Leaf and Spine switches to create a scalable data center fabric optimized for east-west traffic between servers, storage, and compute nodes.

2. What 400G products can be used between Spine and Leaf switches?

The architecture shown includes 400G QSFP-DD SR8, DR4, FR4, DAC and AOC depending on required distance.

3. What is the difference between SR8, DR4 and FR4?

The major difference is the optical architecture and intended reach.

In the illustrated design:

• SR8 supports short-reach links up to approximately 100m
• DR4 supports links up to approximately 500m
• FR4 supports links up to approximately 2km

4. When should 400G DAC be used?

DAC is well suited to very short links where low power, low latency and cost efficiency are important.

5. When should 400G AOC be used?

AOC is useful when more distance or easier cable routing is required than passive DAC can provide while maintaining an integrated plug-and-play cable design.

6. Can QSFP-DD connect to OSFP?

Yes, when the optical/electrical specifications and network protocol are compatible. The diagram shows several Leaf-to-Server examples using QSFP-DD on one side and OSFP on the other.

7. What is a typical 400G Leaf-to-Server solution?

The diagram includes:

• 400G QSFP-DD SR8 to 400G OSFP SR8
• 400G QSFP-DD SR4 to 400G OSFP SR4
• 400G QSFP-DD DR4 to 400G OSFP DR4

8. Can Optech help design the optical BOM?

Yes. Customers can provide the topology, switch/NIC models, number of ports, link distances and preferred fiber infrastructure, and Optech can help identify appropriate optical modules and cables for project evaluation.

9. Can Optech customize 400G products?

Yes. Optech can support custom coding, labels, serial numbers, cable lengths, connector requirements, platform compatibility and other project-specific requirements.

10. Is this architecture suitable for AI clusters?

Yes. Spine-Leaf topology is particularly relevant to AI and HPC because these environments generate large amounts of east-west traffic between servers and accelerators.

Conclusion

A modern 400G AI network requires the right connectivity technology at every layer.

For short distances, DAC, AOC, SR4 and SR8 can provide efficient connectivity.

For medium distances, 400G DR4 can extend links to approximately 500 meters.

For longer links, 400G FR4 can provide up to approximately 2km connectivity.

At the same time, mixed QSFP-DD and OSFP environments allow switches, servers, and high-performance NICs to be connected using the form factor best suited to each platform.

With a broad portfolio of 400G optical transceivers, DAC, AOC, QSFP-DD, OSFP and customized connectivity solutions, Optech can support customers from architecture planning and sample qualification through BOM creation and volume deployment.

Optech welcomes 400G AI, HPC, cloud and data center Spine-Leaf projects worldwide.