net.ipv6.conf.all.accept_ra=2 does not work

Hello everyone,
      I'm trying to configure my machine as an IPv6 router. When I set
net.ipv6.conf.all.forwarding=1, the default routes get deleted, which
is expected. But setting net.ipv6.conf.all.accept_ra=2 does not help
either. Set it before or after net.ipv6.conf.all.forwarding=1, the
default routes WILL get deleted and I've to add it manually. I've even
tried setting these in sysctl.conf and rebooted with the same effect.