# cat /etc/redhat-release

CentOS release 6.3 (Final)

##### host A
# cat /etc/sysconfig/network-scripts/ifcfg-tun0

DEVICE=tun0
BOOTPROTO=none
ONBOOT=no
TYPE=GRE
PEER_OUTER_IPADDR=192.168.103.133
PEER_INNER_IPADDR=172.23.0.1
MY_INNER_IPADDR=172.23.0.2

# ifup tun0


##### host B
# cat /etc/sysconfig/network-scripts/ifcfg-tun0

DEVICE=tun0
BOOTPROTO=none
ONBOOT=no
TYPE=GRE
PEER_OUTER_IPADDR=192.168.102.63
PEER_INNER_IPADDR=172.23.0.2
MY_INNER_IPADDR=172.23.0.1

# ifup tun0