I have a doubt and I hope someone can help me to clarify it.
I have two hosts that are asymmetically routed between them. Traffic from A to B enters firewall 1 through interface Inside, and then reaches B going out on interface Outside. Then, return traffic from host B goes back to A through firewall 1 entering on interface Outside, but due to an old static route, goes out firewall 1 on interface Old_Inside. The path taken after this is different, and there is an additional firewall in the middle.
My first thought was this is not gonna work. But surprisingly it works. I was expecting that the second firewall will see a packet from B to A without having a session established and then it would drop it. I set a capture there to see that, but it is not capturing anything.
My guess is that traffic from B to A is not going out firewall 1 through Old_Inside, and that actually it is going out through Inside. The reason for that would be that the firewall doesn't perform a route lookup for the returning traffic. It just forward it based on the session that is established.
Is this correct?