Just to note,

> 	(2) # ifconfig p2p0 inet6 A B prefixlen 128 alias
> 	(3) # ifconfig p2p0 inet6 A prefixlen 64

> 	(2) and (3) works happily.

(2) works only if the address A is the same as other non-P2P interface,
or the address A is not covered any other prefixes on the node, while
(3) should be used in this case.  Otherwise, some other configuration
like proxy ND is required for neighbor discovery to work.  It should
be called as "Multilink Subnets"...

Atsushi Onoe