The question is why... have not worked with any OS that I needed to manually enter MAC address
The issue is that:
1. Your server is in a restricted network environment, for example in a hosting providers datacenter, and thus has highly probably MAC address restrictions to avoid spoofing the MAC address to look like other servers in that DC (which would cause issues all over the place).
2. previously the kernel set it to one of the Linux bridge actual network ports, that's normally also the one the hosting provider whitelists by default
3. the systemd version that got pulled in with Proxmox VE 7.0 now changed their
MACAddressPolicy to also include virtual devices like Linux bridges or bonds, and generates a unique but persistent MAC address, based on interface-name (e.g.,
vmbr0) and the machine-id (
/etc/machine-id).
So, there's no need from Proxmox VE point of view to change the MAC address, but your hosting provider or network admin requires it - an alternative would thus be to tell them the (additional) MAC address of the bridge they should add to their "allow MAC address" list.
As having a different MAC on the bridge can actually be a good thing for deciding where traffic comes from, but the world seems not to be ready for that yet
As it seems that there are quite some setups in restricted environments that do not know of that restriction, and also some, that know of it but using a mediocre hosting environment that does not provide iKVM/IPMI by default, we will set the bridge MAC address explicitly to the port in the Proxmox VE installer to avoid that.
