Explanation
IPsec uses the Internet Key Exchange (IKE) protocol to negotiate and establish secured site-to-site or remote
access virtual private network (VPN) tunnels. IKE is a framework provided by the Internet Security
Association and Key Management Protocol (ISAKMP) and parts of two other key management protocols,
namely Oakley and Secure Key Exchange Mechanism (SKEME).
In IKE Phase 1 IPsec peers negotiate and authenticate each other. In Phase 2 they negotiate keying materials
and algorithms for the encryption of the data being transferred over the IPsec tunnel.
Source: Cisco Official Certification Guide, The Internet Key Exchange (IKE) Protocol, p.123