The error message "%CRYPTO-6-IKMP_MODE_FAILURE: Processing of Main Mode failed with peer at 150.150.150.1" indicates that IKE Phase 1 negotiation (Main Mode) failed. The most common cause of this failure is a mismatch in ISAKMP (Internet Security Association and Key Management Protocol) policies between the VPN peers.
For IKE Phase 1 to succeed, both VPN devices must have matching ISAKMP policies, including:
Encryption method (e.g., AES, 3DES)
Hashing algorithm (e.g., SHA-256, SHA-1)
Authentication method (e.g., pre-shared key or RSA signature)
Diffie-Hellman group (e.g., DH Group 14, 19, etc.)
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