Blindhog has a good tutorial video up on his site on GRE/IPSec VPN’s. Give it a quick look, his tutorials are always good.

“I first discovered GRE with IPSec about 4 years ago when a customer needed to transmit IPX and multicast traffic over his VPN. Since then, I have primarly used GRE/IPSEC tunnels for transmitting internal routing protocols over the internet. RIP, EIGRP or OSPF can be used over a GRE tunnel just as though it were a point to point circuit. It is also very useful for multicast music on hold…”

Full article here