A Palo Alto Networks Next-Generation Firewall (NGFW) must be configured to use a Layer 3 underlay network in order to secure traffic in a Cisco ACI environment. A Layer 3 underlay network is a physical network that provides IP connectivity between devices, such as routers, switches, and firewalls. A Palo Alto Networks NGFW must use a Layer 3 underlay network to communicate with the Cisco ACI fabric and receive traffic redirection from the Cisco ACI policy-based redirect mechanism. A Palo Alto Networks NGFW does not need to be deployed as a member of a device cluster, receive all forwarding lookups from the network controller, or be identified as a default gateway in order to secure traffic in a Cisco ACI environment, as those are not valid requirements or options for firewall integration with Cisco ACI. Reference: Palo Alto Networks Certified Software Firewall Engineer (PCSFE), [Deploy the VM-Series Firewall on Cisco ACI], [Cisco ACI Underlay Network]