If the address is found in an interface table and the address is not associated with the port on which it was received, the bridge forwards the frame to the port associated with the address. Packets with a source of X and destination of Y are received and discarded, since the computer Y is directly connected to the LAN A, whereas packets from X with a destination of Z are forwarded to network B by the bridge. Broadcast and Multicast Bridges forward a broadcast frame out of all connected ports except that on which the frame was received. The normal action for multicast frame is to treat them as broadcast frame. This is clearly suboptimal, since a bridge may send multicast frames to parts of the network for which there are no interested receivers.
Some bridges implement extra processing to control the flooding of multicast frames.
In either case, the size of the table is finite, and usually constrained to 1000's - 10 000's of entries. In a large LAN this may be a limit. To help keep the table small, most bridges maintain a check of how recently each address was used. This has the effect of removing unused entries, but if the address is again used, before a frame is received from the same source, it will require the frame to be flooded to all ports. A useful side effect of deleting old addresses is that the bridge interface table records only working MAC addresses.