https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/sec_conn_ike2vpn/configuration/15-mt/sec-flex-vpn-15-mt-book/sec-cfg-ikev2-flex.pdf

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/sec_conn_vpnips/configuration/xe-3s/sec-sec-for-vpns-w-ipsec-xe-3s-book/sec-ipsec-virt-tunnl.pdf

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/sec_conn_ike2vpn/configuration/xe-3s/sec-flex-vpn-xe-3s-book/sec-cfg-flex-serv.html#GUID-DB69AED1-1068-4700-B837-5F2967CCD9D2

https://www.cisco.com/en/US/technologies/tk583/tk372/technologies_white_paper0900aecd8029d629.html

https://learningnetwork.cisco.com/thread/127085

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/sec_conn_vpnips/configuration/xe-3s/sec-sec-for-vpns-w-ipsec-xe-3s-book/sec-ipsec-virt-tunnl.html#GUID-334CA827-6CC6-4743-9DC8-D4F8F31F79B5