/etc/network/interfaces.d/51-cloud-init

auto eth0
iface eth0 inet6 static
mtu 1500
address 2001:1337:6969:1100::123
netmask 64
 
# routing
post-up /sbin/ip -6 route add 2001:1337:6969:1100::1 dev eth0
post-up /sbin/ip -6 route add default via 2001:1337:6969:1100::1 dev eth0
pre-down /sbin/ip -6 route del default via 2001:1337:6969:1100::1 dev eth0
pre-down /sbin/ip -6 route del 2001:1337:6969:1100::1 dev eth0

systemctl restart networking.service