The USG5360 works in transparent mode. Its upstream device is a router, and the downstream device is a switch. The router on which sub-interfaces are enabled serves as the gateway of the PC connected to the switch. VLAN 2 is enabled on the firewall, and the port mode is trunk.
After the firewall is connected to the network, the network becomes unreachable.
Change the VLAN ID 2 to 20 on the firewall. The problem is rectified.
The problem may be caused by incorrect configuration of upstream and downstream port modes. However, the check result indicates that both the upstream and downstream ports work in trunk mode. The comparison between the switch configuration, router configuration, and firewall configuration reveals that the VLAN ID on the switch and that on the router are both 20 while the VLAN ID on the firewall is 2.
The VLAN ID 20 of packets sent from the switch to the firewall does not match the VLAN ID 2 of the firewall, so the firewall discards these packets.