What is bridge_fd?

Solution 1:

This is the forwarding delay for interfaces joining the bridge. It's how long it'll be before the interface will be able to do anything. . During this time the bridge will be discovering other bridges and checking that no loops are created. For a better description and the reason for it you need to read up on spanning tree protocol.

Solution 2:

They correspond to brctl settings. Look at man brctl.

brctl(8) can be used for configuring certain spanning tree protocol parameters. For an explanation of these parameters, see the IEEE 802.1d specification. The default values should be just fine. If you don’t know what these parameters mean, you probably won’t feel the desire to tweak them.