Dell S5212F-ON to Cisco C9500-48Y4C: dual-coded FS SFP28 DACs stay protocol down while 2 m Cisco/Cisco cables link

Between VxRail Dell S5212F-ON TOR switches and Cisco C9500-48Y4C switches, FS.com SFP28 DACs behaved inconsistently: a 5 m cable custom-coded Cisco on one end and Dell on the other came Line Up, Protocol down; a 2 m Cisco/Cisco-coded cable linked; a 3 m cable would not link even with the 2 m cable's configuration cloned. Loopbacks on the Cisco side worked with every cable except the Cisco/Dell one, and reprogramming the Dell end of the 5 m cable to Cisco coding made it work. The poster asked whether SFP port power requirements vary with cable length. No replies.

• Hardware: Dell S5212F-ON, Cisco Catalyst C9500-48Y4C
• Transceivers: FS.com SFP28 DAC 2 m / 3 m / 5 m (custom-coded)
• Form factor: SFP28, DAC

Symptoms / messages:

Line Up, Protocol down
3 m DAC never links
works after recoding the Dell end as Cisco