Multiple macs self-assigned IP on ethernet (wifi okay though)
I'll answer and accept this question as I'm certain now that the problem is the interaction between modern Macs and my Cisco switch. I can kind of fix the problem by:


Setting the Mac ethernet "hardware" tab to manual 1000T/full duplex
Setting corresponding switch ports to 1000T/full duplex
Turning off the switch green ethernet facility
Making sure switch is at the latest firmware release


This seems to work well until the Mac sleeps, then the connection is lost again. I'm still trying to work out how to work around that issue.