Comment # 6 on bug 894889 from Marius Tomaschewski

IPv6 routes are set up via ipv6 router RA -- as soon as a RA is there, there
will be a default route assigned to the sender (==router).
This happens regardless of further contents of the RA (prefix which may allow
autoconf or not, further routes which an RA may contain, ra.manged=on flag
which allows DHCPv6 client to request IPv6 address or other-config=on which
does not allow this [DNS, NTP, ... config only]).