Although Main mode is more secure, you must select Aggressive mode if there is more than one dialup Phase 1 configuration for the interface IP address, and the remote VPN peer or client is authenticated using an identifier local ID. Aggressive mode might not be as secure as Main mode, but the advantage to Aggressive mode is that it is faster

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ISAKMP (IKE Phase 1) status messages MM_WAIT_MSG# - …

Networking Fundamentals: IPSec and IKE - Cisco Meraki