Answer by Brian for Making changes to default IPv6 route persistent in CentOS 7

One place you can put it is in the /sbin/ifup-local file which is run when an interface comes up.

#!/bin/shif [[ "$1" == "eth0" ]]then  ip -6 route del default  ip -6 route add default via 2001:470:xxxx:xx::1 src 2001:470:xxxx:xx::6else  #DO_NOTHINGfi

It doesn't exist by default so you will need to create it and set permissions to make it executable.