IPSec over GRE Tunnel on IOS - How to Configure Two Cisco Routers to Create Site-to-Site VPN Tunnel - UniNets Blog
Configure Site to Site IPSec VPN Tunnel in Cisco IOS RouterMustBeGeek
Setting up an IPsec tunnel between Teltonika Networks and Cisco devices - Teltonika Networks Wiki
Site-to-Site IKEv2 IPSec VPN using Pre-Shared Key Authentication - simple configuration example for two Cisco routers - NIL - Network Information Library