To support asymmetric routing with redundancy in a legacy environment using Palo Alto Networks firewalls, the following two features must be enabled:

Policy-based forwarding (PBF): This feature allows you to direct traffic to a specific interface or next-hop IP address, bypassing the normal routing table. PBF is crucial for managing asymmetric routing scenarios where inbound and outbound traffic might take different paths.

HA active/active: High Availability (HA) active/active configuration enables two firewalls to simultaneously manage traffic, providing redundancy and failover capabilities. This setup is essential for maintaining network availability and performance in environments with asymmetric routing, as it ensures that both firewalls can actively handle traffic.

References:

Palo Alto Networks High Availability (HA) Guide

Palo Alto Networks Policy-Based Forwarding Documentation