Thanks for the fast response Ryan!  

Here's some information from a fully updated CentOS 7.6 as of today, unmodified cloud-init 18.2-1.el7.centos.2 but no NetworkManager on AWS with IPV6 enabled.

```
[root@ip-172-31-21-249 ~]# cat /etc/sysconfig/network
# Created by cloud-init on instance boot automatically, do not edit.
#
NETWORKING=yes
NETWORKING_IPV6=yes
IPV6_AUTOCONF=no

[root@ip-172-31-21-249 ~]# ps -ef |grep dhc
root      1347     1  0 00:14 ?        00:00:00 /sbin/dhclient -1 -q -lf /var/lib/dhclient/dhclient--ens5.lease -pf /var/run/dhclient-ens5.pid -H ip-172-31-21-249 ens5
root      1425     1  0 00:14 ?        00:00:00 /sbin/dhclient -6 -1 -lf /var/lib/dhclient/dhclient6--ens5.lease -pf /var/run/dhclient6-ens5.pid ens5 -H ip-172-31-21-249

[root@ip-172-31-21-249 ~]# cat /var/lib/dhclient/dhclient6--ens5.lease
default-duid "\000\001\000\001$\263\360\355\002\030CCi:";
lease6 {
  interface "ens5";
  ia-na "CCi:" {
    starts 1562458400;
    renew 75;
    rebind 120;
    iaaddr 2600:1f14:589:f00:1212:8330:638:9075 {
      starts 1562458400;
      preferred-life 150;
      max-life 450;
    }
  }
  option dhcp6.client-id 0:1:0:1:24:b3:f0:ed:2:18:43:43:69:3a;
  option dhcp6.server-id 0:3:0:1:2:42:8f:b:a5:8e;
}
lease6 {
  interface "ens5";
  ia-na "CCi:" {
    starts 1562458400;
    renew 75;
    rebind 120;
    iaaddr 2600:1f14:589:f00:1212:8330:638:9075 {
      starts 1562458400;
      preferred-life 150;
      max-life 450;
    }
  }
  option dhcp6.client-id 0:1:0:1:24:b3:f0:ed:2:18:43:43:69:3a;
  option dhcp6.server-id 0:3:0:1:2:42:8f:b:a5:8e;
}
lease6 {
  interface "ens5";
  ia-na "CCi:" {
    starts 1562458477;
    renew 75;
    rebind 120;
    iaaddr 2600:1f14:589:f00:1212:8330:638:9075 {
      starts 1562458477;
      preferred-life 150;
      max-life 450;
    }
  }
  option dhcp6.client-id 0:1:0:1:24:b3:f0:ed:2:18:43:43:69:3a;
  option dhcp6.server-id 0:3:0:1:2:42:8f:b:a5:8e;
}
lease6 {
  interface "ens5";
  ia-na "CCi:" {
    starts 1562458552;
    renew 75;
    rebind 120;
    iaaddr 2600:1f14:589:f00:1212:8330:638:9075 {
      starts 1562458552;
      preferred-life 150;
      max-life 450;
    }
  }
  option dhcp6.client-id 0:1:0:1:24:b3:f0:ed:2:18:43:43:69:3a;
  option dhcp6.server-id 0:3:0:1:2:42:8f:b:a5:8e;
}


[root@ip-172-31-21-249 ~]# ip -6 a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 state UNKNOWN qlen 1000
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: ens5: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9001 state UP qlen 1000
    inet6 2600:1f14:589:f00:1212:8330:638:9075/64 scope global dynamic
       valid_lft 442sec preferred_lft 142sec
    inet6 fe80::18:43ff:fe43:693a/64 scope link
       valid_lft forever preferred_lft forever

[root@ip-172-31-21-249 ~]# ip -6 route show
unreachable ::/96 dev lo metric 1024 error -113 pref medium
unreachable ::ffff:0.0.0.0/96 dev lo metric 1024 error -113 pref medium
unreachable 2002:a00::/24 dev lo metric 1024 error -113 pref medium
unreachable 2002:7f00::/24 dev lo metric 1024 error -113 pref medium
unreachable 2002:a9fe::/32 dev lo metric 1024 error -113 pref medium
unreachable 2002:ac10::/28 dev lo metric 1024 error -113 pref medium
unreachable 2002:c0a8::/32 dev lo metric 1024 error -113 pref medium
unreachable 2002:e000::/19 dev lo metric 1024 error -113 pref medium
2600:1f14:589:f00::/64 dev ens5 proto kernel metric 256 expires 432sec pref medium
unreachable 3ffe:ffff::/32 dev lo metric 1024 error -113 pref medium
fe80::/64 dev ens5 proto kernel metric 256 mtu 9001 pref medium
```


If you ever want to play around with this yourself, I periodically create and publish AMIs based on the template I reference: https://github.com/irvingpop/packer-chef-highperf-centos7-ami#latest-amis