Intel X552 (FreeNAS) to Mellanox ConnectX-3 (ESXi) over Mellanox DAC: both ends see no carrier

A FreeNAS box with on-board Intel X552 SFP+ ports (Xeon D) and an ESXi 6.7 host with a Mellanox ConnectX-3 (MT27500, Dell Y3KKR) were cabled with two Mellanox MCP2104-X003B passive DACs from 2014. Looping each device to itself brought the ports up, but between the two systems ESXi showed Disconnected and FreeNAS reported No Carrier. The tunables hw.ixgbe.unsupported_sfp and hw.ix.unsupported_sfp were set, auto-negotiation was disabled on ESXi and forced via mediaopt on FreeNAS, all without effect. The thread received no replies.

• Hardware: Intel X552, Mellanox ConnectX-3 MT27500 (Dell Y3KKR)
• Transceivers: Mellanox MCP2104-X003B DAC 3 m
• Form factor: SFP+, DAC

Symptoms / messages:

No Carrier
ESXi: Disconnected
loopback works but cross-link does not