strongswanでIKEv2/IPsec のVPNを接続すると、下記のようにローカル間で通信可能となります。

tokyo[10.10.0.0/16] <---> osaka[172.16.0.0/24]

yum install strongswan

iptables -I INPUT -p udp --dport 4500 -j ACCEPT
iptables -I INPUT -p udp --dport 500 -j ACCEPT
iptables -t nat -A POSTROUTING -j MASQUERADE

firewall-cmd --permanent --zone=public --add-service=ipsec
firewall-cmd --permanent --zone=public --add-port=4500/udp
firewall-cmd --reload

sysctl -w net.ipv4.ip_forward=1

conn %default
    type=tunnel
    authby=psk
    keyingtries=3
    keyexchange=ikev2
    ike=aes256-sha256-modp2048
    esp=aes256-sha256-modp2048

conn osaka
    leftid=@tokyo
    leftsubnet=10.10.0.0/16
    rightid=@osaka
    right=200.200.0.200
    rightsubnet=172.16.0.0/24
    auto=start

conn %default
    type=tunnel
    authby=psk
    keyingtries=3
    keyexchange=ikev2
    ike=aes256-sha256-modp2048
    esp=aes256-sha256-modp2048

conn tokyo
    leftid=@osaka
    leftsubnet=172.16.0.0/24
    rightid=@tokyo
    right=100.100.0.100
    rightsubnet=10.10.0.0/16
    auto=add

tokyo

iptables -t nat -I POSTROUTING 1 -s 10.10.0.0/16 -m policy --dir out --pol ipsec -j ACCEPT

osaka

iptables -t nat -I POSTROUTING 1 -s 172.16.0.0/24 -m policy --dir out --pol ipsec -j ACCEPT

tokyo

firewall-cmd --direct --add-rule ipv4 nat POSTROUTING 0 -s 10.10.0.0/16 -m policy --dir out --pol ipsec -j ACCEPT

osaka

firewall-cmd --direct --add-rule ipv4 nat POSTROUTING 0 -s 172.16.0.0/24 -m policy --dir out --pol ipsec -j ACCEPT

[root@vpn-test ~]# strongswan status
connecting to 'unix:///var/run/charon.ctl' failed: Connection refused
failed to connect to stroke socket 'unix:///var/run/charon.ctl'

# systemctl stop strongswan
# rm -rf /var/run/charon.ctl
# systemctl start strongswan