In networks with asymmetric routes, such as in a dual ISP environment, connectivity
issues occur when traffic arrives at one interface on the firewall
and leaves from another interface. If the route is asymmetrical,
where the forward (SYN packet) and return (SYN/ACK) paths are different,
the firewall is unable to track the state of the entire session
and this causes a connection failure. To ensure that the traffic
uses a symmetrical path, which means that the traffic arrives at
and leaves from the same interface on which the session was created,
you can enable the
Symmetric Return option.