my config on NetBSD.
on linux (another side) quite similar with numbers reversed and route to
my subnet not default.

anyway it's STRANGE pppd can't directly set addresses except link-local
ones

/etc/ppp/peers/stalka:
dty03 28800 modem crtscts
+ipv6
debug
netmask 255.255.255.255
persist noauth nolog defaultroute nobsdcomp
noproxyarp
maxfail 0
mtu 1496
mru 1496
ipparam stalka

/etc/ppp/ipv6-up:
#!/bin/ksh
ifcmd=""
rcmd="add"
if [ `basename $0` == ipv6-down ];then
 ifcmd="delete"
 rcmd="delete"
fi
if [ $6 == stalka ];then
 /sbin/ifconfig $1 inet6 3ffe:b80:ab0::1 3ffe:b80:ab0:ffff::1 $ifcmd
 /sbin/route $rcmd -inet6 default 3ffe:b80:ab0:ffff::1
fi


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http://www.3miasto.net/~wojtek/pgp.txt