Comment 4 for bug 1126560

> Interfaces are brought up as they show up from the kernel, so they can indeed be brought up in parallel.

I have tested this and it seems to be false (assuming there is a bug on booting). I have written a script which helps me debugging this problem and placed it in /etc/network/if-pre-up.d/ and /etc/network/if-up.d/. The script delays itself to 5 seconds and logs at which PID it is currently running. So it is possible to say in which order ifupdown has called the scripts and if they are running sequentially or parallel.

network_reboot.log contains the information what happens after a system reboot. In this log if-pre-up.d and if-up.d were called parallelized 3 times.
network_restart contains the information what happens after calling /etc/init.d/networking restart. In this log if-pre-up.d and if-up.d were called sequentially 2 times.

Because I'm having only 2 network devices I'm assuming the second variant is the correct one. It would be a little easier to know for which network device the scripts were running. But I have already opened a feature request for this (https://bugs.launchpad.net/ubuntu/+source/ifupdown/+bug/1128403).