If your Linux box gets its IPv6 routing information via router advertisments, you may have noticed that upon enabling forwarding, it disregards those advertisments and it loses the configured gateways.
The have this configuration working again, set
accept_ra 2 in
/etc/network/interfaces. This value is not documented in the
interfaces manual, but since kernel version 2.6.37 it allows the interface to both do forwarding and accept router advertisments.