Port Splitting on Mellanox SN2100, SN2700 and SN2410: 4x25G and 2x50G Breakouts Without Losing a Port

On NVIDIA (Mellanox) Spectrum switches, a 100G port splits into 4 × 25G with a QSFP28-to-4×SFP28 breakout such as the MCP7F00-A002R30N, or into 2 × 50G with the MCP7H00-G002R30N. The catch depends on the model. On the SN2100, every port can split with no side effects. On the SN2700, only top-row ports split four ways, and doing it blocks the bottom-row port below. On the SN2410, ports 49/51/53/55 split four ways and disable 50/52/54/56. Any port on these switches can split two ways.

Below: the per-model rules, the Onyx and Cumulus commands, the cables, and the NICs for the far end.

Split Rules by Model

• SN2100 (16 × 100G, half width): every QSFP28 port splits to 4 × 10/25G or 2 × 50G. No ports are blocked.
• SN2700 (32 × 100G): top-row ports split to 4 × 10/25G, and the bottom-row port beneath each one is blocked. Any port splits to 2 × 50G. The most 25G ports you can get is 16 top ports × 4 = 64.
• SN2410 (48 × 25G + 8 × 100G): ports 49, 51, 53 and 55 split to 4 × 10/25G, which disables 50, 52, 54 and 56. Any QSFP28 port splits to 2 × 50G.

These rules come from NVIDIA's SN2000 hardware manual. The SN2700 blocking rule catches people most often: cable a server into a bottom port, split the port above it, and the server loses its link.

Configure the Split

Onyx (MLNX-OS): shut the port, change its module type, then configure the new sub-interfaces:

interface ethernet 1/1 shutdown
interface ethernet 1/1 module-type qsfp-split-4 force
interface ethernet 1/1/1-1/1/4 speed 25G force

Use qsfp-split-2 for 2 × 50G. Splitting resets the interface configuration, so re-apply VLANs and MTU afterwards. Command forms vary slightly across Onyx releases; check the Ethernet Interfaces chapter of the manual for your version.

Cumulus Linux / NVIDIA NVUE: set the breakout in /etc/cumulus/ports.conf (for example 1=4x25G) and restart switchd, or use nv set interface swp1 link breakout 4x on NVUE releases. The resulting ports appear as swp1s0 to swp1s3.

The Server End

• 25G: MCX4121A-ACAT (refurbished, $54.95) or MCX512A-ACAT (refurbished, $149.95). Run one leg from each of two switches to give a server a redundant pair.
• 10G only: MCX4121A-XCAT (refurbished, $54.95). SFP28 cages, 10GbE only. Pair it with a 4 × 10G breakout.
• 50G: any 100GbE QSFP28 ConnectX-5, such as the MCX516A-CCAT (refurbished, $279.95), set to 50G, on an MCP7H00 leg.

Once the legs show up, FEC is the usual reason a 25G link won't come up: the FEC mismatch checklist. For cable lengths and vendor-coded breakouts, see QSFP28 breakout cables: 4x25G vs 4x10G.

FAQ

Can every port on an SN2700 split into 4 × 25G?
No. Per NVIDIA's SN2000 hardware manual, only the top-row QSFP28 ports split into 4 × SFP28 (10/25G), and the bottom-row port below a 4-way split is blocked. Any port can split into 2 × 50G without blocking a neighbor.

Can every port on an SN2100 split?
Yes. Every SN2100 QSFP28 port can split into 4 × 10/25G or 2 × 50G, and NVIDIA lists no blocked ports as a result.

Which SN2410 ports split into 4 × 25G?
Ports 49, 51, 53 and 55. Ports 50, 52, 54 and 56 are disabled while the port next to them runs in 4 × SFP28 mode. Any QSFP28 port can split into 2 × 50G.

What cable do I need for a 4 × 25G split?
A 100G QSFP28 to 4 × SFP28 breakout DAC such as the NVIDIA MCP7F00-A002R30N (refurbished, $149.95). For 2 × 50G, the MCP7H00-G002R30N (refurbished, $44.95).

What NIC goes on a 25G breakout leg?
Any SFP28 card: the MCX4121A-ACAT (refurbished, $54.95) or the MCX512A-ACAT (refurbished, $149.95). Both are dual-port, so two legs from two different switches give each server a redundant 25G pair.

My breakout legs are up but won't pass traffic or won't link. What now?
Check FEC first. 25G links need both ends to agree on RS-FEC, FC-FEC or none, and switch and NIC defaults often differ. Our FEC mismatch guide walks through it.

Ready to Buy?

Breakouts plus the server-side NICs. Genuine NVIDIA/Mellanox, lifetime warranty, shipping from Knoxville, Tennessee. Prices below checked October 22, 2026.

• MCP7F00-A002R30N — 100GbE to 4 × 25GbE breakout DAC, 2 m — $149.95, refurbished, in stock at Resilient Tec; NVIDIA-coded for Spectrum switches
• MCP7F00-A002R30N — 100GbE to 4 × 25GbE breakout DAC, 2 m — $199.95, new, in stock at Resilient Tec; new condition
• MCP7H00-G002R30N — 100G to 2 × 50G breakout DAC, 2 m — $44.95, refurbished, in stock at Resilient Tec; 2-way splits
• MCX4121A-ACAT — ConnectX-4 Lx, 2 × 10/25GbE SFP28 — $54.95, refurbished, in stock at Resilient Tec; 25G server end
• MCX512A-ACAT — ConnectX-5, 2 × 10/25GbE SFP28 — $149.95, refurbished, in stock at Resilient Tec; 25G server end with NVMe-oF offload
• MCX4121A-XCAT — ConnectX-4 Lx, 2 × 10GbE SFP28 — $54.95, refurbished, in stock at Resilient Tec; 10G server end

You Might Also Need

• QSFP28 breakout cables — 100G to 4x25G vs 4x10G
• DAC cable no link? — fixing 25G/100G FEC mismatches
• How to read a ConnectX part number — every MCX suffix decoded
• 100G QSFP28 DACs for AI homelab clusters — the ConnectX cable guide
• MTEF-KIT-J explained — rack kits for SN3700, SN4600 and SN4700

Sources

• NVIDIA Spectrum SN2000 Switch Systems Hardware User Manual — Cable installation and port splitting