IPv6 address preferred_lft forever?

On my Ubuntu 14.04.1 I can see ipv6 addresses:

% ip -6 addr

…

inet6 XXXX:XXXX::XXXX:XXXX:XXXX:XXXX/64 scope global dynamic

validlft 2591986sec preferredlft 604786sec

For this address I would like to set preferred_lft forever. I have in my /etc/network/interfaces file:

up /sbin/ip -6 addr add XXXX:XXXX::XXXX:XXXX:XXXX:XXXX/64 dev eth0 preferred_lft 0

up /sbin/ip -6 addr add XXXX:XXXX::XXXX:XXXX:XXXX:XXXX/64 dev eth0 preferred_lft forever

where I changed preferred_lft from 0 to forever. However, after a reboot this has not changed the above output of ip -6 addr ?

It seems the interface file is not being read (properly)?

A ping6 on this address works fine.