Is there an IPv6 router advertising to use DHCPv6 on the network in it's RA
(the managed [=IP + dns,...] or other-config [dns, ... info] bit set) ?

You can install radvd and run "radvdump" while "ip link set up" or for a while
as it is sent periodically to see if there is an RA and what it contains.

When there is no RA [which is also used to set the default IPv6 route], or when
the IPv6 router advertises to not use DHCPv6 (managed off, other-config off] and
you want to override it, set DHCLIENT6_MODE=managed in the ifcfg file.

You can also pass this kernel parameters to _linuxrc_ at installation time:
ifcfg=*=dhcp6,DHCLIENT6_MODE=managed