2016-10-26 Ok, allow-hotplug means some other process has to start the interface11 months ago
Today I rebooted the homeserver greenblatt for kernel updates and noticed PPPoE over VDSL did not come up at all. It turns out allow-hotplug in /etc/network/interfaces which I added after the previous boot to speed up boottime a bit means something else has to trigger the 'ifup' of the interface. So now there is a ifup pppdray & in /etc/rc.local. Interesting datapoint is that all IPv6 bindings came up perfectly so I guess that IPv6 bug is triggered by some race condition in configurations of interfaces.