6
In IPv4, ff-ff-ff-ff-ff-ff is broadcast address so that this frame can pass layer 2 of all machines.
In IPv6, ARP is not used but ICMPv6 is used to know mac-address by multicasting. In this case, what is mac-address of multicasting??
If mac-address is not matched, this frame cannot pass layer 2 of destination machine. So, I really wonder how multicast packet is passed layer 2 by not using ff-ff-ff-ff-ff-ff.
As a side note, the "all nodes" address is in practice treated as broadcast, since MLD is not used for it. – user1686 – 2017-09-29T04:50:52.240