Cumulus Linux vs Onyx: NVIDIA Switch Operating Systems

NVIDIA Spectrum switches are open networking platforms: the switch hardware and the network operating system are separable, so the same physical chassis can be sold running different software. That is why a single platform such as the SN4600C appears under multiple ordering part numbers, and why "which OS" is a purchasing question rather than something you decide after the box arrives.

Historically two operating systems have shipped preloaded on these platforms: Cumulus Linux and Onyx. The industry-standard SONiC also runs on Spectrum hardware, though what you are entitled to is determined by the part number you buy, not by what the silicon can technically boot.

Cumulus Linux is a Linux distribution that happens to run a switch. NVIDIA acquired Cumulus Networks in 2020 and it has since become the default NOS on Spectrum Ethernet platforms.

Its defining characteristic is that it is genuinely Linux rather than a vendor CLI with a Linux kernel underneath. Interfaces, routing, and configuration are managed with tooling a systems engineer already knows, which means the same automation stack used for servers — configuration management, version control, CI-style validation — extends to the network layer instead of requiring a separate discipline.

That is the reason it became the default for large-scale and cloud-style buildouts. When you are standing up many switches rather than a handful, the ability to treat them as fleet infrastructure rather than individually-configured appliances is the operational difference that matters.

Onyx — previously MLNX-OS — is the older Mellanox-lineage switch operating system, predating NVIDIA's acquisition of Mellanox. It follows the more traditional networking model: a vendor-specific CLI and configuration idiom, familiar to engineers coming from conventional enterprise switch platforms.

Onyx is not deficient, and Onyx-based switches in production keep working. But NVIDIA has withdrawn the Onyx ordering part numbers for the SN4600C — the ordering documentation for the SN4000 family now lists only the Cumulus Linux variants for that platform, and the Onyx configurations appear as end-of-life at resellers.

The distinction worth holding onto: what was retired was the ordering configuration, not the switch. The SN4600C platform itself remains in mass production. An Onyx unit is a discontinued configuration of a current product.

On the MSN4600 family the operating system is encoded as a trailing C on the ordering part number:

• MSN4600-CS2RC and MSN4600-CS2FC — trailing C, Cumulus Linux. These are the current part numbers.
• MSN4600-CS2R and MSN4600-CS2F — no trailing C, Onyx. Withdrawn.

Note how little visual difference there is between a current part number and a discontinued one — a single character at the end of a thirteen-character string. Listings frequently drop it, and "MSN4600-CS2R" and "MSN4600-CS2RC" are easy to treat as a typo of one another when they are in fact different products with different lifecycle status.

The letter before that trailing character is a separate axis entirely — it encodes airflow direction, covered in MSN4600-CS2RC vs MSN4600-CS2FC. A part number can be wrong on the operating system, wrong on airflow, or wrong on both.

This is where the operating-system question stops being academic. A switch is hardware plus a software entitlement, and the two do not necessarily travel together through a resale.

On any unit bought outside authorized distribution, three things are worth establishing explicitly rather than assuming:

• The operating-system license. Confirm the unit carries a valid, transferable entitlement to the NOS it is running, not merely an installed image.
• Software update access. An entitlement that does not include updates leaves you unable to take security fixes, which for an infrastructure device with a long service life is a real exposure rather than an inconvenience.
• Support status. Manufacturer support is generally tied to the original purchase and frequently does not follow the hardware to a second owner.

None of these make a secondary-market purchase wrong — a great deal of infrastructure is bought this way, deliberately and successfully. They are simply the questions that determine what you are actually buying, and they are answered before the purchase rather than after. A unit whose OS entitlement does not transfer is a different item at a different price than one whose does.

For a new deployment the question largely answers itself: current SN4600C part numbers ship Cumulus Linux, so that is what you buy unless a specific reason points elsewhere.

The cases for wanting Onyx are narrow and concrete rather than preferential — an existing Onyx fleet whose tooling and runbooks you are extending, or a configuration you need to match exactly. Both are legitimate; neither is a reason to seek out a withdrawn part number for a new buildout, where you would be starting on a discontinued ordering configuration by choice.

If you are matching an existing fleet, the operating system is worth confirming against the running switches rather than the purchase records — units get reimaged, and what an OPN shipped with is not always what a switch is running years later. See the SN4600C specs for the platform behind both, or the networking catalog for current availability.