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400G to 800G Migration: Making the Business Case with Real Numbers
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.
The Three Levers That Drive TCO
Optical network TCO has three components, and 800G shifts all of them:
- Capital cost per gigabit. An 800G transceiver costs roughly 1.5–1.8× a 400G equivalent — but delivers 2× the capacity. Per-gigabit cost drops 15–25%.
- Switch port count. A 32-port 800G switch replaces two 32-port 400G switches. That eliminates an entire switch — chassis, power supplies, optics for the inter-switch links, and half the rack space.
- Power and cooling. An 800G port draws 14–18 W vs 10–14 W for 400G. But because you need half as many ports for the same capacity, total power per Tbps drops 25–35%.
Scenario Analysis: Three Real Migration Profiles
| 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) |
Where 400G Still Makes Sense
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 Real Payback: Port Reduction, Not Per-Gigabit Price
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
