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Doubling lane speeds from 100G to 200G PAM4 cuts port count in half and saves 18 kW per 10K-GPU pod. But not every network needs 800G yet. Here's a practical decision framework.
Apex Group Editorial Team|June 2026|5 min read
Every network architect running a 400G fabric is asking the same question: when do I move to 800G? The answer isn't "as soon as possible" — it depends on your workload, your switch generation, and your CapEx cycle. Here's how to decide.
| METRIC | 400G-DR4 | 800G-DR8 | DELTA |
|---|---|---|---|
| Lanes | 4× 100G PAM4 | 8× 100G PAM4 | 2× bandwidth per port |
| Ports per RU (32-port switch) | 32 | 32 (same cage) | 2× total throughput |
| Power per Port | ~12W | ~16W | +33% per port |
| Power per Gbps | 30 mW/Gbps | 20 mW/Gbps | -33% per Gbps |
| Leaf ports for 10K-GPU fabric | 2,048 | 1,024 | -50% ports |
| Total interconnect power (10K GPU) | ~41 kW | ~23 kW | -18 kW |
| Transceiver Cost (relative) | 1× | ~1.6× | +60% per port |
| Cost per Gbps | 1× | ~0.8× | -20% per Gbps THE KEY INSIGHT800G transceivers cost more per port — but less per gigabit. If you're filling every port anyway (GPU clusters, AI fabrics), 800G saves money. If you're not saturating 400G ports, upgrading is premature. |
Stay on 400G if:
1. Your switches don't support 800G. Check the ASIC. Broadcom Tomahawk 4 = 400G max. Tomahawk 5 / Cisco Silicon One G200 = 800G capable. If your switch silicon can't do 112 Gbps SerDes, 800G optics won't help.
2. Your port utilization is under 50%. If your 400G leaf-spine links average 200 Gbps of actual traffic, you don't need 800G. Wait until utilization consistently exceeds 70%.
3. You're 18+ months from your next switch refresh. 800G optics require 800G-capable switch ASICs. If you just bought 400G switches, run them for their depreciation cycle. The 800G optics will be cheaper when you refresh.
4. Your workload is general enterprise, not AI/ML. Standard enterprise traffic patterns (email, databases, VDI) don't saturate 400G pipes. Upgrade when AI training or HPC workloads arrive.
Move to 800G if:
1. You're building a GPU cluster > 1,000 GPUs. AI training's all-reduce traffic saturates 400G ports at 60-80% of theoretical max. 800G halves the leaf port count and saves real power.
2. You're out of front-panel ports. When your spine switch runs out of cages, 800G doubles throughput without adding switches.
3. You're refreshing switches anyway. If a Tomahawk 5 or equivalent purchase is on the roadmap, spec 800G optics from day one. Don't buy 400G optics that you'll replace in 18 months.
4. Power budget is your constraint. Colocation cages often cap power at 5-10 kW per rack. Moving from 400G to 800G frees 18 kW for GPU density in a 10K-GPU pod.
You don't have to go all-800G overnight. Because QSFP-DD cages accept both 400G and 800G optics, you can migrate pod by pod:
| PHASE | ACTION | BENEFIT |
|---|---|---|
| Phase 1 | Upgrade spine to 800G (spines auto-negotiate to 400G per leaf port) | Zero leaf changes. Spine bandwidth doubled. |
| Phase 2 | Upgrade GPU leaf pods to 800G SR8/DR8 | GPU fabric runs at full 800G. Enterprise leaves stay at 400G. |
| Phase 3 | Full 800G fabric + 800G ZR+ for DCI | Maximum density, minimum per-bit power and cost. BOTTOM LINEIf you're building an AI cluster today, go 800G — the per-Gbps savings justify it. If you're running enterprise workloads on 400G switches with > 2 years of depreciation left, wait. And if you're refreshing switches anyway, spec 800G optics from day one — the worst outcome is buying 400G optics 6 months before an 800G switch refresh. |
