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If you see that Phase 1 IKE SA process done but still get below [info] log message,
please check ZyWALL/USG and FortiGate Phase 2 Settings. Both ZyWALL/USG and
FortiGate must use the same Protocol, Encapsulation, Encryption, Authentication
method and PFS to establish the IKE SA.
MONITOR > Log
Make sure the both ZyWALL/USG and FortiGate security policies allow IPSec VPN
traffic. IKE uses UDP port 500, AH uses IP protocol 51, and ESP uses IP protocol 50.
Default NAT traversal is enable on ZyWALL/USG, please make sure the remote
IPSec device must also have NAT traversal enabled.