Direct answer
Do not purchase from “400G to four 100G links” alone. Cisco documents an SR4.2-to-SR1.2 architecture example, but a project still needs exact modules, current host/software support and a verified fiber map. Equal Ethernet rates do not make SR4, SR4.2 and other optical types interchangeable.
Definition and application
The relevant application is incremental multimode migration: upgrading one end while retaining matching 100G connections at the other. Cisco describes duplex bidirectional 100G SR1.2 and an associated SR4.2 breakout architecture. Cisco migration overview
The overview was updated on June 6, 2023. It explains architecture, not current stock or universal platform support. Its migration examples are not proof that every existing cable plant can remain unchanged.
| Review item | Required information | Insufficient substitute |
|---|---|---|
| 400G end | Full module and interface definition | Abbreviation alone |
| 100G ends | Specified compatible optical types | Any multimode 100G module |
| Host | Current support and breakout configuration | Matching port form factor |
| Cable | Mapping, polarity and fiber requirements | Total fiber count |
| Existing route | Connections, length and test record | Previous successful operation |
Procurement workflow
Freeze the optical combination, then review current host support and configuration. Draw the full physical route with each connector and destination port. Evaluate reach and loss using the exact module documentation; do not fill missing part numbers with generic distance assumptions.
Acceptance should verify every branch and the operational end-to-end route. Ask a reuse proposal to identify retained, replaced and added components explicitly.
Procurement FAQ
- Can every 400G module provide four 100G links? No; optics and host support both matter.
- Are SR4 and SR4.2 the same? Do not procure them as interchangeable types.
- Is existing duplex cabling always reusable? It depends on the complete topology and link conditions.
- Is ordering a breakout cable enough? Confirm optics, host configuration and acceptance too.
- What if the module is unknown? Obtain the exact part number before specifying the assembly.
Links: 400G optical types, breakout checklist, MPO harness path. No compatibility claim is implied.
Sources checked: 2026-10-05