Firewall

A firmware upgrade finished cleanly, the firewall came back healthy, and the next morning two of a customer’s LANs could not reach each other. The traffic between them had crossed one firewall interface with no policy at all, passed by the old firmware and dropped by the new one under implicit deny. Here is how that flow stayed invisible, the Fortinet default that matches the symptom, the read-only checks that find it before you upgrade, and why the explicit policy is a better fix than the one-line setting.