In transparent mode, Cisco Secure Firewall Threat Defense (FTD) operates as a Layer 2 firewall, acting as a "bump in the wire" without performing routing functions. This mode allows devices on either side of the firewall to communicate as if they were on the same network. To achieve this, the firewall typically uses bridge domains or similar configurations to group devices.
A bridge domain groups interfaces in transparent mode so that devices on each side of the firewall can communicate directly. Ensuring that both servers are in the same bridge domain allows traffic to pass through the firewall transparently, enabling communication between the two servers as if the firewall were not there.