Please be informed that ESP is a layer3 protocol and it doesn't have any port number. So for the ESP protocol port forwarding is not possible. Also if you are trying to establish a vpn tunnel and Nat device is in between, in this scenario you need to enable NAT-T on both peer ends and the port UDP 4500, UDP 500 needs to be allowed on the NAT device placed in between to allow the vpn traffic to pass through. For more details kindly check below link:
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