A script to label the secondary ipv6 addresses differently, thus making the first ipv6 the default for outgoing connections

outgoing_ipv6.sh

#!/bin/bash

ETH=$(ip -6 r | awk '/^default/ {print $5}')

for IP in $(awk -F '=' '/IPV6ADDR_SECONDARIES/ {gsub("\"",""); gsub("/[0-9]*","/128"); print $2}' /etc/sysconfig/network-scripts/ifcfg-${ETH})
do
  ip addrlabel add prefix ${IP} label 1001
done

In CentOS6, you can run this script after network startup by adding it in /sbin/ifup-local