4.2.a Implement and troubleshoot IPsec with preshared key

http://rayas-security.blogspot.com/2013/06/ipsec-vpn-main-mode-vs-aggressive-mode.html

4.2.a [i] IPv4 site to IPv4 site

IPSec between two sites such as branch and a headquarter is known as site to site or LAN to LAN tunnel. It can be configured with or without GRE. IKE has two modes of operation, aggressive or main mode. Main mode hides IKE/ IPSec peer identities.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4827-4829).  . Kindle Edition.

http://www.cisco.com/en/US/docs/ios-xml/ios/sec_conn_vpnips/configuration/xe-3s/sec-ipsec-virt-tunnl.html