Transparent firewall mode forwards traffic at Layer 2, so the participating physical interfaces must be assigned to at least one bridge group. This allows the device to inspect traffic while remaining logically transparent to the connected switch stacks. Bridge Protocol Data Units should also be exempted from advanced inspection. BPDUs are essential for spanning-tree operation, and delaying or blocking them during inspection restarts can produce topology instability or an interruption. A BVI provides Layer 3 addressing for device-originated traffic or management but is not inherently required merely to inspect transit traffic. Jumbo- frame configuration is unnecessary unless the network explicitly uses jumbo frames, and separate data and management routes do not establish transparent forwarding. Cisco specifically recommends using an EtherType ACL to trust BPDUs on each bridge-group member interface. Cisco transparent firewall configuration

Transparent mode is a firewall configuration in which the firewall acts as a "bump in the wire" or a "stealth firewall" and is not seen as a router hop to connected devices. In transparent mode, the firewall can forward traffic at both layer 2 and layer 3 for the same subnet, as it does not perform any address translation or routing. The firewall inspects the traffic and applies security policies based on the source and destination IP addresses, ports, and protocols. Transparent mode is useful when you want to deploy a firewall without changing the existing network topology or addressing scheme1.