DMVPN Tunnels | Using NHRP and GRE to Create Scalable Tunnels
Introduction to DMVPN
Cisco Certifications - Dynamic Layer 3 VPNs with Multipoint GRE Tunnels Configuration Example This document describes how to configure Dynamic Layer 3 (L3) VPNs with the multipoint Generic Routing Encapsulation (mGRE) Tunnels
IPSec over GRE Tunnel on IOS - How to Configure Two Cisco Routers to Create Site-to-Site VPN Tunnel - UniNets Blog