You would use a GRE tunnel in case you want to encapsulate other protocols in the communication between sites. It basically creates you a virtual point to point link, hiding the details for the network between both points.
I have used before for customers who only need to know about the "hub". After implementing GRE, I always configure IPSEC over the tunnel for encryption.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...