Cisco IOS VPN Configuration
Scenario 1: Gateway-to-gateway with preshared secrets
http://www.vpnc.org/InteropProfiles/cisco-ios.txt
IPSEC example
http://www.priscilla.com/ipsecexample.htm
An Introduction to IPSEC Security Encryption
http://www.cisco.com/en/US/tech/tk583/tk372/technologies_tech_note09186a0080094203.shtml