The mapping on the VM-Series Firewall remains the same no matter which vNICs you add on ESXi. No matter which interfaces you activate on the firewall, they always take the next available vNIC on ESXi. In the following example, eth3 and eth4 on the VM-Series Firewall are paired to vNICs 2 and 3 on ESXi respectively. If you add want to add two additional interfaces, you must activate vNICs 4 and 5; doing this requires you to power down the VM-Series firewall. If you activate eth1 and eth2 on the VM-Series Firewall, the interfaces will reorder themselves. This can result in a mapping mismatch and impact traffic.