Before deploying a VM-Series into AWS, you should configure three subnets (management, untrust, and trust) as well as three interfaces corresponding to those subnets. Attach those interfaces to the VM-Series as you are deploying it - eth0 (first interface configured) should be on the management network. I usually make eth1 my untrust interface and eth2 is my trust interface. When the instance comes up, ethernet1/1 in the firewall maps to eth1 and ethernet1/2 maps to eth2. If you configure the interfaces in the firewall management GUI to match the configuration in the AWS portal, you should be ready to go. You can also set the interfaces to DHCP and they should get the appropriate IP addresses assigned automatically.
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