Cisco GRE and IPSec - GRE over IPSec - Selecting and Configuring GRE IPSec Tunnel or Transport Mode

This article aims to explain the differences between GRE IPSec Tunnel and GRE IPSec Transport mode and how they can be configured and implemented in a Secure Cisco VPN Network. Packet Analysis of both modes with detailed diagrams and Cisco IOS configuration commands, ensures the reader will not be left with any unanswered questions on this topic!

http://www.firewall.cx/cisco-technical-knowledgebase/cisco-routers/872-cisco-router-gre-ipsec-tunnel-transport.html

Understanding VPN IPSec Tunnel Mode and IPSec Transport Mode - What's the Difference?

This article aims to help clear the difference between IPSec Tunnel and IPSec Transport mode. Each IPSec mode is examined using real examples and packet-level illustrations to ensure both advantages and disadvantages are clearly understood. We also examine IPSec AH & ESP encapsulation-encryption differences for each IPSec mode.

http://www.firewall.cx/networking-topics/protocols/870-ipsec-modes.html