Unlocking 25 Gigabit/S on 10 GbE Direct Attach Copper

9 hours ago

The author upgraded their homelab setup to 25 Gigabit Ethernet but found that existing DAC cables only supported 10 Gbps.
DAC (Direct Attach Copper) cables are passive and cheaper than active optical cables but require replacement for higher speeds.
SFP connectors separate the 'what' from the 'how' in networking, allowing flexibility in transmission technology.
RJ45 Ethernet cables are limited to 10 Gbps and consume more power compared to DAC cables.
SFP connectors include metadata like vendor ID and product ID, which can restrict compatibility with certain hardware.
Enterprise networking gear often enforces vendor lock-in, requiring expensive programming devices to modify SFP metadata.
The author used a Banana Pi BPI-R3 router to modify the EEPROM of a DAC cable, changing its reported speed from 10 Gbps to 25 Gbps.
By adjusting the nominal signaling rate in the EEPROM and fixing checksums, the author successfully 'uprated' the cable.
Testing with iperf3 confirmed the modified cable achieved speeds over 22 Gbps, close to the 25 Gbps target.
Not all 10 Gbps DAC cables can be uprated; physical differences like cable thickness and PCB design affect compatibility.
The author explored the internals of a non-upgradable DAC cable, revealing differences in construction compared to upgradable ones.

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