FD refers to forwarding delay. The bridge_fd option sets the delay before the interface will be ready. During the delay, the bridge tries to discover other bridges and checks that there are no network loops if STP is on. By default, the forwarding delay is set to 0, as shown in the following code:

bridge_fd 0 

In most cases, the default value of 0 is enough. In a very complex virtual environment with several dozen bridges, increasing this number to 3 or 4 might help. Without this delay, the bridge will start transmitting data packets regardless of whether the other destination bridge is available or not. Increasing the delay time allows the source bridge to check all the bridges and not transmit any data if the destination bridge is down, thus preventing unnecessary network bandwidth consumption.