Phase 1 of the two-phase negotiation process conducted by IKE operates in ______ mode.

 Phase 1 of the two-phase negotiation process conducted by IKE operates in Main mode or Aggressive mode12. Main mode is more secure than Aggressive mode, as it protects the identities of the peers and uses six messages to establish the IKE SA13. Authentication, Quick, and High Alert are not valid modes for IKE phase 1.

References: Understand IPsec IKEv1 Protocol, Internet Key Exchange for IPsec VPNs Configuration Guide, Internet Key Exchange