Weirdest L1 issue - 'long' DACs cause CRC errors

[FishCustard]

I've just today finished tracking down one of the oddest Layer 1 issues I've ever seen - our shiny new HP Aruba switches don't like 5m DACs. Using them causes a massive flood of CRC alignment errors on the uplink ports, causing the ports to vacillate on and off. The upshot is that network throughput on said switch understandably drops from the gigabit range to ~10 Mbps. Use 3m cables and everything is fine.

 

This has to be a bad batch of 5m cables right? (I've not managed to find any compatible 5m ones other than the original batch) Has anyone else ever seen anything like this? Do 'DAC testers' exist, so that I can prove once and for all WTF is going on here? Our installers are replacing the cables with 3m ones, so there's no urgency, I just like to understand exactly what is going on.

[Davit2005]

We had same issue with Aruba 2540's it seems the ports on the switches are not powerfull enough and suffer with cables above 3m. We do use longer cables between Chassis type switches though and these work OK.

[FishCustard]

We had same issue with Aruba 2540's it seems the ports on the switches are not powerfull enough and suffer with cables above 3m. We do use longer cables between Chassis type switches though and these work OK.

 

Interesting! Perhaps our 2930s have the same issue.