FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
IPsec VPN failed to established when Sonicwall pointing to dynamic IP [i.e FortiDDNS]. Debug output on FortiGate shows, after second message is received by initiator 'ignoring unencrypted INVALID-COOKIE' and retransmit.
To address this issue, on Sonicwall side change the Peer ID [IPV4 Address] to be the FortiGate's private IP which is facing the NAT Router.
