Aggressive Mode - VPN and IPSec tutorial
For a successful and secure communication using IPSec, the IKE (Internet Key Exchange) protocols takes part in a two step negotiation. Main mode or Aggressive mode (Phase 1) authenticates and/or encrypts the peers. Quick mode (Phase 2) negotiates the algorithms and agree on which traffic will be sent across the VPN. Below I discuss Aggressive mode (Phase 1).
Aggressive mode can be used within the phase 1 VPN negotiations, as opposed to Main mode. Aggressive mode takes part in fewer packet exchanges. Aggressive mode does not give identity protection of the two IKE peers, unless digital certificates are used. This means VPN peers exchange their identities without encryption (clear text). It is not as secure as main mode, but the advantage to aggressive mode is that it is faster than Main mode.
Aggressive mode is typically used for remote access VPN’s (remote users). Also you would use aggressive mode if one or both peers have dynamic external IP addresses. Although you don’t have to use Aggressive mode if the peer devices are using digital certificates.
Wikipedia's guide to IKE (Internet Key Exchange)