Evaluating Co-Packaged Optics Switches for AI Data Centers

Co-packaged optics (CPO) integrates optical engines directly into the switch ASIC package, dramatically reducing power consumption and latency compared to traditional pluggable optics. This guide walks you through a structured evaluation of CPO switches for an AI data center, covering technical criteria, testing procedures, and operational considerations.

What You’ll Need

• Switch vendor datasheets and architecture documents
• Network traffic generator (e.g., IXIA, Spirent) capable of 800G or 1.6T per port
• Optical power meter and spectrum analyzer
• Thermal chamber or controlled environment for temperature testing
• Access to a small test rack with power and cooling
• Software tools for monitoring switch telemetry (e.g., SNMP, gNMI)
• Budget for at least two vendor evaluation units

Step-by-Step

1. Define your AI workload requirements. Identify the dominant traffic patterns: distributed training (all-to-all), inference (many-to-one), or storage (east-west). Quantify required bandwidth per rack, latency budget (typically under 1 microsecond for AI fabrics), and power density limits.
2. Shortlist vendors based on port speed and density. CPO switches typically offer 51.2 Tbps or 102.4 Tbps in a 1U or 2U form factor. Verify the number of optical ports (e.g., 64 OSFP ports at 800G) and whether they support breakouts to lower speeds (e.g., 2x400G).
3. Compare power efficiency. Look for the switch’s maximum power draw (in watts) and calculate power per bit (W/Gbps). CPO should show a 30-50% reduction vs. pluggable optics. Check the power envelope for the entire chassis, including optical engines, and ensure it fits your rack power budget.
4. Assess optical performance. For each port, measure transmitter and receiver optical power, extinction ratio, and bit-error rate (BER) at the specified reach (typically 2m for DAC, up to 2km for SMF). Use a clean fiber with known loss. Verify that the BER is below 1e-15 for 800G per lane.
5. Test thermal behavior. Run a sustained 100% traffic load for 24 hours in a thermal chamber at your data center’s maximum ambient temperature (e.g., 40°C). Monitor the switch’s internal temperature sensors and optical engine temperatures. Ensure no thermal throttling or packet loss occurs.
6. Evaluate latency and jitter. Use a traffic generator to measure cut-through latency for various frame sizes (64B, 256B, 1518B). Compare against the vendor’s claimed latency. Jitter should be minimal (<1 microsecond). For AI workloads, also measure latency under bursty traffic patterns.
7. Verify reliability and fault tolerance. Pull an optical fiber from a live port and confirm the switch detects the fault, isolates it, and does not impact other ports. Test link failover to a redundant path. Check the mean time between failures (MTBF) and the vendor’s field failure data.
8. Check software and automation support. Confirm the switch supports the APIs you need (e.g., OpenConfig, gNMI) for telemetry and provisioning. Test integration with your orchestration system (e.g., Kubernetes with CNI). Verify support for VXLAN, EVPN, and RoCEv2 with PFC/ECN.
9. Review serviceability and lifecycle. CPO optics are not field-replaceable; the entire switch must be sent back for repair. Understand the vendor’s RMA process, replacement time, and whether they offer a spare pool. Check the product’s expected lifecycle and upgrade path.
10. Run a proof-of-concept in your own environment. Deploy the switch in a small AI cluster (e.g., 8 GPUs) and run a realistic training job (e.g., a transformer model). Monitor throughput, loss, and latency. Compare against your current pluggable optics solution.

Common Mistakes

• Ignoring cable types. CPO switches often require specific fiber types (e.g., single-mode for 2km reach). Using multimode fiber will cause high BER or no link.
• Overlooking the power envelope per port. Some vendors quote low power per port but exclude the optical engine power. Always ask for total chassis power.
• Testing only at room temperature. AI data centers often run hot. If you skip thermal testing, you may encounter premature failures.
• Assuming pluggable compatibility. CPO ports may not accept standard pluggable transceivers. Verify that the switch supports the optics you plan to use.
• Not checking software maturity. CPO is new; some features (e.g., advanced telemetry) may be incomplete. Test the exact features you need.

Tips

• Ask vendors for reference customers and independent test reports.
• Run your evaluation for at least 30 days to catch intermittent issues.
• Use a standardized test plan (e.g., from OCP or TIA) to compare vendors fairly.
• Consider total cost of ownership: CPO may have higher upfront cost but lower power and cooling costs over 5 years.
• Plan for spare units. Since optics are not replaceable, a failure means the whole switch goes back.

Troubleshooting

• Link does not come up: Check fiber polarity, cleanliness, and whether the port is configured for the correct speed. Use a power meter to verify received power is within range.
• High BER or packet loss: Clean the fiber connectors. Check for excessive loss in the link (e.g., bad splice). Verify that the switch’s firmware is up to date.
• Thermal shutdown: Ensure the switch’s airflow direction matches your rack cooling. Check for blocked vents. Increase fan speed manually if supported.
• Latency spikes: Look for buffer congestion or ECN marking. Check if PFC is configured correctly. Disable any power-saving features that may introduce jitter.
• Incompatibility with existing optics: CPO switches may require specific cable types (e.g., MTP/MPO). Use a breakout cable if needed. If the switch only supports CPO, you may need a media converter.

Frequently Asked Questions

What is the main benefit of co-packaged optics?

CPO reduces power consumption by 30-50% and latency by 20-30% compared to pluggable optics, because the optical engine is closer to the ASIC, eliminating the retimer and high-speed electrical traces.

Can I use existing pluggable transceivers with a CPO switch?

Usually no. CPO switches have optical engines embedded in the package, so they do not accept pluggable transceivers. You must use the switch’s built-in optical ports with appropriate fiber cables.

How does CPO affect maintenance?

Optical engines are not field-replaceable. If a port fails, the entire switch must be replaced. This makes spare switches critical and RMA turnaround time a key factor.

Is CPO ready for production AI data centers?

As of 2024, CPO is still early but is being deployed in some large-scale AI clusters. It is most suitable for greenfield deployments where the entire network is new and can be optimized for CPO.

What is the typical power consumption of a CPO switch?

A 51.2 Tbps CPO switch typically draws 500-700W, compared to 700-1000W for a similar pluggable optics switch. Exact numbers vary by vendor and port density.

How do I test CPO switch reliability?

Run a 72-hour burn-in test with full traffic at elevated temperature. Monitor for bit errors, optical power drift, and any thermal alerts. Also, verify the switch meets the vendor’s MTBF claim.

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