IB 2x100GB splitter cable only cable "1" works MCP7H50

I’m having an odd issue. I have a QM8790, MCP7H50-H IB splitter cables, and 2 hosts with MT4123 (single port HDR100) HCAs. All have latest firmware installed on switch and HCAs. For some reason, only the host with cable “1” connects. Other cable, “2”, polls. I’ve swapped cables, hosts, tested with several cables. Only cable “1” of the splitter cable works. The other doesn’t. What is going on?

Here’s some more details of cable “2” (blue tab):
mlxlink -d mlx5_0

Operational Info

State : Polling
Physical state : ETH_AN_FSM_ENABLE
Speed : N/A
Width : N/A
Loopback Mode : No Loopback
Auto Negotiation : ON

Supported Info

Enabled Link Speed : 0x00000075 (HDR,EDR,FDR,QDR,SDR)
Supported Cable Speed : 0x0000007f (HDR,EDR,FDR,FDR10,QDR,DDR,SDR)

Troubleshooting Info

Status Opcode : 2
Group Opcode : PHY FW
Recommendation : Auto-negotiation no partner detected.

Hello jdvw,

Thank you for posting your inquiry on the NVIDIA Developer Forum - Infrastructure and Networking - Section.

For both links of the Y-cable to function, you need to make sure that the switch is configured for split_mode.

The QM8790 is an unmanaged switch. For configuring split_mode on this switch, please follow the procedure described in the follow link → Using mlxconfig - NVIDIA Docs

You only need to set the switch to split_mode. After that the switch will auto-sense the cable and will activate the 2nd link.

Thank you and regards,
~NVIDIA Networking Technical Support

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