Apparently, this issue was caused by the host having high utilisation on
the CPU on host boot. This caused the guests to not have their
interfaces configured in time. The link on an interface can be up while
the interface itself is not yet fully initialised. This causes the
networking-services to accept neighbor advertisements and router
advertisements which can add IPv6 routes to the routing table.

When the interface is almost complete, it will set the default route as
I had a gateway rule in my interfaces file. This fails (file exists)
because there already is a default IPv6 route for this particular
gateway, assigned via RA (router advertisement). To solve this issue, I
simply removed the gateway from the interfaces file, as the gateway is
auto assigned via RA. Another fix would be to disable accepting RA on
this particular interface, the default value or all interfaces by using:

pre-up net.ipv6.conf.device.accept_ra=0

where "device" is "all", "default" or the actual device name (eth0, em0
etc.).

I hope this can help some people suffering from the same problem as I
had. It took me way too long to find the cause of this problem. The
actual fix was found on this link:
http://unix.stackexchange.com/questions/306139/rtnetlink-answers-file-
exists-after-adding-ipv6-address.

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https://bugs.launchpad.net/bugs/495394

Title:
  autostart fails on boot time host when network devices not ready

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