Configuring Site to Site IPSec VPN Tunnel Between Cisco Routers
Cisco IOS VPN Configuration Guide - Site-to-Site and Extranet VPN Business Scenarios [Cisco 7200 Series Routers] - Cisco
Site-to-Site IKEv2 IPSec VPN using Pre-Shared Key Authentication - simple configuration example for two Cisco routers - NIL - Network Information Library
Configure a Site-to-Site VPN Tunnel with ASA and Strongswan - Cisco