Comment # 6 on bug 1166358 from Marius Tomaschewski

You're not on a host but on a router, see comment 4:

  net.ipv6.conf.all.forwarding = 1
  net.ipv6.conf.br0.forwarding = 1

A router (kernel) ignores RAs and does not autoconfigure the interfaces,
so all the autoconf=1, ... have no effect.

Please either configure the kernel to process the RA using:

  net.ipv6.conf.all.accept_ra = 2

(see
https://www.mjmwired.net/kernel/Documentation/networking/ip-sysctl.txt#1463) in
/etc/sysctl.d/... or /etc/sysconfig/network/ifsysctl[-br0]
or to not use the along with auto (=follow RA) mode, but use the
following in your ifcfg file:

  DHCLIENT6_MODE=managed
  DHCLIENT6_ADDRESS_LENGTH=64

The RA provides the following "bits" for the statefull autoconfiguration
alias dhcp6:

AdvManagedFlag     on   ## DHCLIENT6_MODE=auto -> managed: get IP + DNS,NTP
or
AdvOtherConfigFlag on   ## DHCLIENT6_MODE=auto -> info: get DNS,NTP config only

AdvDefaultLifetime > 0  ## causes to add an ipv6 default route (dhcp6 does not
have options for routing)

prefix ${prefix}/${prefix_length} {
  AdvOnLink on;         ## usable in dhcp6 for ip a a $IP/${prefix_length}
}

Without the prefix-length and an On-Link hint in the RA, dhcp6 is mandated
by RFC5942 to assume $IP/128 -- in combination with missing default route
due to discarded RA, you're not even able to reach hosts in your local
network...