How can I configure a GRE tunnel with IPv6 inner addresses?

Issue

• Why are IPv6 inner addresses ignored when configuring a GRE tunnel as described in article How to setup a GRE tunnel that survive reboots??

• After creating the following configuration files for a GRE tunnel IPv6 inner addresses are ignored and only IPv4 addresses are configured:

  [root@rhel6tunnel1 ~]# cat /etc/sysconfig/network-scripts/ifcfg-gre1 
  DEVICE=gre1
  TYPE=GRE
  ONBOOT=yes
  MY_INNER_IPADDR=10.0.0.1
  PEER_OUTER_IPADDR=172.16.128.2
  PEER_INNER_IPADDR=10.0.0.2
  BOOTPROTO=none
  IPV6INIT=yes
  IPV6ADDR=2001:db8::1/64
  
  [root@rhel6tunnel2 ~]# cat /etc/sysconfig/network-scripts/ifcfg-gre1
  DEVICE=gre1
  TYPE=GRE
  ONBOOT=yes
  MY_INNER_IPADDR=10.0.0.2
  PEER_OUTER_IPADDR=172.16.128.1
  PEER_INNER_IPADDR=10.0.0.1
  BOOTPROTO=none
  IPV6INIT=yes
  IPV6ADDR=2001:db8::2/64