The question facing most AI data center teams is not whether to move to 800G — it is how to get there without breaking what is already running. This playbook walks through a four-phase approach that starts with your spine layer, uses breakout configurations to maintain backward compatibility, and ends with a full 800G fabric ready for the eventual move to 1.6T.
Table of Contents
12 comprehensive sections — jump to any topic- 1Why Phased Migration Matters
- 2Phase 1: Audit and Readiness (Wks 1-4)
- 3Switching Platform Inventory
- 4Fiber Plant Verification
- 5Power and Cooling Headroom
- 6Phase 2: Spine Upgrade with Breakout (Wks 5-10)
- 7Why Spine-First Works
- 8Phase 3: Leaf Migration (Wks 11-20)
- 9Phase 4: Full 800G Fabric (Wks 20-24)
- 10Common Migration Risks
- 11Migration Checklist
- 12Vitex Portfolio and Support
1. Why Phased Migration Matters
The question facing most AI data center teams is not whether to move to 800G — it is how to get there without breaking what is already running. A 400G leaf-spine fabric carrying production GPU training jobs cannot tolerate a forklift upgrade. The migration has to happen in phases, each one validated before the next begins, with the fabric running mixed speeds throughout.
Audit and readiness assessment
Spine upgrade with 2x400G breakout
Leaf migration with native 800G
Full 800G fabric, 1.6T-ready
2. Phase 1: Audit and Readiness Assessment (Weeks 1–4)
Before ordering any 800G hardware, you need a clear picture of what you have and what needs to change. The audit covers switching platforms, fiber infrastructure, power and cooling capacity, and software readiness.
3. Switching Platform Inventory
Document every switch model, NOS version, ASIC generation, and port configuration. Identify which switches support 800G line rates and breakout modes. Broadcom Tomahawk 4-based switches are 400G-native and need replacement. Tomahawk 5 and later support 800G.
Needs Replacement
- Broadcom Tomahawk 4 — 400G-native, no 800G support
- Any switch without 800G line rate capability
- Platforms without breakout mode support
- End-of-support NOS versions incompatible with 800G config
800G-Ready
- Broadcom Tomahawk 5 — 51.2 Tbps, native LPO SerDes
- Broadcom Tomahawk 6 — 102.4 Tbps, full LPO support
- NVIDIA Spectrum-4 — 51.2 Tbps, IHS and RHS OSFP
- Arista 7800R, Cisco 8000 series with 800G line cards
4. Fiber Plant Verification
800G DR8 transceivers require single-mode fiber. If any inter-switch trunks are multimode (OM3/OM4), those paths need re-cabling. Run OTDR tests on every trunk to verify insertion loss, connector quality, and continuity. Test MPO polarity — MPO-16 connectors require Type-C polarity. A mismatch tolerable at 100G will cause failures at 800G.
5. Power and Cooling Headroom
Each 800G transceiver draws 14–17W (DSP) or 7–8.5W (LPO). A 32-port switch fully loaded with DSP modules adds 544W in optics alone. Calculate the per-rack power delta between your current 400G modules (8–12W each) and the 800G replacements, then verify PDU and cooling capacity.
← swipe to scroll →| Module Type | Per-Module Power | 32-Port Switch Total | Delta vs 400G (10W avg) |
|---|---|---|---|
| 400G DSP (current) | 8–12W | 320W (avg) | Baseline |
| 800G DSP | 14–17W | 544W | +224W per switch |
| 800G LPO | 7–8.5W | 272W | +~48W per switch |
| 800G LRO | ~9W | 288W | +~64W per switch |
6. Phase 2: Spine Upgrade with Breakout (Weeks 5–10)
Replace spine switches one at a time with 800G-capable platforms, but do not touch leaf switches. Each new 800G spine port uses a 2x400G breakout cable to connect to two existing 400G leaf uplinks.
The procedure: drain traffic by adjusting routing weights, swap hardware, install 800G transceivers with MPO-16 to 2xMPO-12 breakout cables, bring up interfaces in 2x400G mode, validate ECMP hashing and routing convergence, then run production traffic for 48–72 hours before the next spine.
The result: where you previously had 32 spine ports to 32 leaf uplinks, you now have 32 spine ports to 64 leaf uplinks. Spine bandwidth doubles with zero changes to the leaf layer.
7. Why Spine-First Works
The spine layer is the natural starting point because spine switches handle only transit traffic — they do not connect directly to servers. A spine swap affects only uplink paths, protected by ECMP redundancy. Traffic redistributes across remaining spines automatically. No server-facing ports go down.
Spine-First Advantages
- Spine switches carry only transit traffic — no direct server connections
- ECMP redundancy redistributes traffic across remaining spines automatically
- No server-facing ports go down during spine swap
- 2x400G breakout maintains full backward compatibility with leaf layer
- Each spine can be drained, swapped, and validated independently
Phase 2 Validation Steps
- Drain traffic by adjusting routing weights before hardware swap
- Install 800G transceivers with MPO-16 to 2xMPO-12 breakout cables
- Bring up interfaces in 2x400G mode — verify all breakout links
- Validate ECMP hashing and routing convergence
- Run production traffic for 48–72 hours before next spine
8. Phase 3: Leaf Migration with Native 800G (Weeks 11–20)
With all spines running 800G, begin replacing leaf switches in pairs. Drain traffic, replace hardware, bring up native 800G uplinks, and validate. As each leaf upgrades, replace the breakout cable on the spine side with a native 800G MPO-16 trunk.
The fabric runs a mix of native 800G and 2x400G breakout links during this phase. This mixed-speed state is fully supported by modern NOS platforms and ECMP routing. Server-facing ports run at whatever speed servers support — NICs do not need to change yet.
9. Phase 4: Full 800G Fabric (Weeks 20–24)
All spine-to-leaf links run native 800G over MPO-16 trunks. Remove breakout cables, consolidate cabling, and update monitoring for 800G telemetry: per-lane FEC error rates, module temperature, transmit power, and receiver SNR.
Your fiber plant is now 1.6T-ready. The same single-mode fiber and MPO-16 connectors will carry 1.6T when next-generation ASICs arrive. No re-cabling required.
Phase 4 Completion Criteria
- All spine-to-leaf links running native 800G over MPO-16 trunks
- All breakout cables removed and replaced with MPO-16 trunks
- Monitoring updated for 800G telemetry: per-lane FEC, module temp, TX power, RX SNR
- Cabling plant documented and labeled for 800G topology
1.6T Readiness
- Same OS2 single-mode fiber carries 1.6T without re-cabling
- MPO-16 connectors are 1.6T-compatible — no connector changes needed
- Only transceivers change when next-generation ASICs arrive
- Fiber plant investment is fully protected through 1.6T generation
10. Common Migration Risks
| Risk | Impact | Mitigation |
|---|---|---|
| MPO polarity mismatch | Link failure after swap | OTDR + visual fault locator on every trunk before cutover |
| Cooling headroom exceeded | Thermal throttling | Measure ambient at module height; budget 14–17W per port |
| NOS breakout config error | Ports fail to come up | Lab-validate NOS version and breakout CLI before production |
| Form factor mismatch | Module will not insert | Verify IHS vs RHS per platform; order samples first |
| ECMP rebalancing | Temporary asymmetry | Drain spine before swap; validate hash distribution post-swap |
11. Full Migration Checklist
12. Vitex Portfolio and Migration Support
Vitex provides 800G transceivers, breakout cables, and MPO-16 infrastructure for every phase of the migration. See our Interconnect Selection Guide for per-phase recommendations.
← swipe to scroll →| Migration Phase | Vitex Products | Notes |
|---|---|---|
| Phase 1 — Audit | OTDR test support, MPO polarity verification | Pre-migration fiber validation services |
| Phase 2 — Spine upgrade | 800G DR8 or 2xDR4 OSFP (IHS/RHS), MPO-16 to 2xMPO-12 breakout cables | IHS for QM3400; RHS for CX-8; both for Spectrum-4 |
| Phase 3 — Leaf migration | 800G SR8 or DR4 OSFP, native MPO-16 trunk cables | Replace breakout cables as each leaf upgrades |
| Phase 4 — Completion | MPO-16 structured cabling, cassettes, labeling kits | Final cabling consolidation and 1.6T-ready infrastructure |
