I just tested, and the IPv6 ND packets are being sent to the MGID ff12:401b:ffff:0:0:0:ffff:ffff. This makes sense because net/ipv6/ndisc.c uses dev->broadcast in ndisc_mc_map() if it doesn't know about the interface type. I'll see if creating ipv6_ib_mc_map() helps. - R.