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The technical case for 800G is clear — double the capacity per port, same QSFP-DD cage, same fiber plant. But technical capability does not pay invoices. The migration from 400G to 800G has to work on a spreadsheet before it works in a data center. Here is what the numbers actually look like across three common deployment scenarios.
Optical network TCO has three components, and 800G shifts all of them:
| Metric | Scenario A: Greenfield Spine-Leaf | Scenario B: Brownfield Capacity Add | Scenario C: AI Cluster Expansion |
|---|---|---|---|
| Starting point | New build, 400G or 800G decision | Existing 400G fabric, adding capacity | 10K GPU cluster, doubling to 20K |
| Switch count (32-port, 51.2T) | 400G: 64. 800G: 32 | Add 16 × 800G to existing 400G | 400G: 128. 800G: 64 |
| Optics cost delta | 800G saves ~22% per Gbps | 800G saves ~18% on new ports | 800G saves ~25% per Gbps |
| Power savings | ~28% less total power | ~20% less incremental power | ~32% less total power |
| Rack space | Half the racks | No space saving | Half the racks |
| 3-year TCO savings | ~$1.2M (800G vs 400G) | ~$380K (new capacity only) | ~$2.8M (800G vs 400G) |
Not every link should be 800G. Three situations where staying at 400G is the right financial call:
Low-utilization access ports. Server-to-leaf links where the server NIC is 100G or 200G. Putting an 800G DR8 on a server that generates 25 Gbps of actual traffic wastes 97% of the port capacity.
Brownfield fabric with limited growth. If your existing 400G spine-leaf fabric has 30%+ headroom and traffic growth is under 15% annually, the migration payback stretches past 5 years.
Metro DCI with distance constraints. 400G ZR+ coherent modules are more mature, have a broader multi-vendor ecosystem, and cost 30–40% less per module than 800G ZR+. For DCI links under 4 Tbps total demand, 10 × 400G channels are often cheaper than 5 × 800G channels.
The financial case for 800G is not about the transceiver price — it is about eliminating switch ports, switches, and inter-switch links. A 32-port 800G switch consolidates two 400G switches, saving not just the second switch but also the 8–16 spine-to-leaf links that connected them. Those inter-switch optics alone often cost $15,000–$30,000 per link.
Decision framework: If your migration eliminates switch ports (greenfield, technology refresh, cluster expansion), 800G delivers 20–30% TCO savings over 3 years. If you are adding capacity to an existing 400G fabric without reducing switch count, the savings drop to 5–10%. 400G remains the right choice for access ports where NIC speeds cap utilization below 200 Gbps.
APEX Group supplies the full 400G and 800G portfolio — QSFP56-DD, QSFP-DD DR8/FR4/SR8, and coherent CFP2-DCO / QSFP-DD ZR+ — so network architects can build mixed-speed fabrics that put 400G on access and 800G on spine/interconnect, optimizing TCO at every tier.
APEX GROUP — www.apexallinone.com
