I am unable to get a 100G connection between a DGX Spark and a UACC-OM-QSFP28-SR4 module. I cannot establish a 100G link between a DGX Spark and a UACC-OM-QSFP28-SR4.
I have tested various settings, including all 100G_4x variants, of course — without success. I have tested multiple of these transceivers and cables, also without success.
A 40G connection works, and a 100G DAC connection works as well.
Did I configure something incorrectly, or does anyone have an idea what else I could try?
Setup:
UACC-OM-QSFP28-SR4
UACC-OFC-SA-MPMP-5M
UACC-OM-QSFP28-SR4
ECS Aggregation (UniFi)
Original installation with all updates applied up to today.
This is a known compatibility issue, not a configuration error on your part.
The core problem: The DGX Spark uses a ConnectX-7 NIC (Mellanox/NVIDIA), and these adapters are notoriously picky about which optics they’ll negotiate 100G with. The UACC-OM-QSFP28-SR4 is a Ubiquiti-branded QSFP28 SR4 transceiver, and it’s almost certainly not on NVIDIA’s validated optics list. The ConnectX-7 firmware enforces strict vendor validation for high-speed links.
Why 40G works but 100G doesn’t: At 40G (4×10G NRZ), the signal integrity requirements are much more forgiving, and the NIC often falls back to 40G as a “safe” mode when it can’t properly handshake at 100G (4×25G NRZ) with an unrecognized transceiver. The DAC works at 100G because passive copper cables don’t have firmware/EEPROM vendor-ID negotiation issues the same way active optics do.
What to try:
First, check what the ConnectX-7 actually reports. SSH into the Spark and run sudo mlxlink -d /dev/mst/mt4129_pciconf0 -m (or whichever mst device corresponds to your port). This will show you the module info, link status, and importantly any error flags like “unsupported module” or “high power not granted.”
Also try sudo mlxconfig -d /dev/mst/mt4129_pciconf0 q | grep ALLOW to see if there’s an ALLOW_NON_MLNX_CABLE parameter. If it exists and is set to false, you can try setting it to true with mlxconfig and rebooting — this sometimes unlocks third-party optics at 100G.
The most reliable fix: Use Mellanox/NVIDIA-branded QSFP28 SR4 modules (MMA1B00-CS4 or similar). These are guaranteed to negotiate 100G with ConnectX-7. You can also try FS.com QSFP28-SR4 modules — they’re much cheaper and generally work well with Mellanox NICs since FS programs compatible vendor codes.