I have two identical new EA-8500 routers running LEDE, and as a shortcut to getting them both configured, I backed up the config from one and installed it to the other. To my surprise, the eth0 mac address for both units is now the same - even though => there are no mac address overrides in the configurations!
I know I can override one or both addresses with a custom mac address but I'm curious why this happened: I assumed there were some network adapter hardware bits that were used to guarantee uniqueness for the portion after the OUI bits.
Is the ULA prefix somehow used as a hash seed for the local mac addresses? That is in the config files, and is therefore the same..
I'd suggest to start with resetting the router to its defaults (firstboot) for testing, if you still get the same MAC address, there most likely is an issue with extracting the MAC addresses from ubootenv/ ART. Contrary to ordinary x86 gear, embedded devices like router usually don't have an EEPROM with a hardconfigured MAC address, but rather save those expenses for ondevice EEPROMs and move those settings into the main flash chip.
Firmwares like LEDE are now tasked with the problem of finding out where and how the vendor is storing this data and then to set it during device boot (unless u-boot already does the right thing, which is often not the case). Unfortunately problems like this aren't always immediately obvious, as affected devices typically revert to a (common) fallback MAC address and keep working fine, until you happen to have an identical model running in the same network segment and get to notice duplicate MAC addresses.
Thanks, that helps explain it. I built both images from source and installed the identical images via tftp. I might just generate locally administered unique addresses myself to ensure no problem down the road.