Waste few hours to configure L2TP VPN on Debian. To reduce Googling time when I need to do it again.
Found some almost correct ways on following links:
Few things need modify:
use ipsec up/down to control IPsec tunnel use correct pipeline when start L2TP connection: $ sudo ipsec up vpn-name
$ echo "c vpn-name " | sudo tee /var/run/xl2tpd/l2tp-control
And use ike-scan to find VPN server’s supported protocols. Debug using https://doc.pfsense.org/index.php/IPsec_Troubleshooting if IPSec tunnel can’t be establish.