the forward/filter conundrum...

given:

the three functions of  switching:

1) learning: switches and bridges remember the hardware address of each incoming frame on an interface and update the mac database with this information. the mac database is also known as a forward/filter table

2) forward/filter: upon receiving a frame the switch examines the destination hardware address, finds the exit interface in the mac database and forwards the frame out that corresponding interface

3) avoiding loops : in a redundant network, STP is used to stop network loops while not impeding that redundancy

this is good until...

The following list provides a quick review of the basic logic a switch uses:

1.A frame is received.

2.If the destination is a broadcast or multicast, forward on all ports except the port on which the frame was received.
3.If the destination is a unicast, and the address is not in the address table, forward on all ports except the port on which the frame was received.
4.If the destination is a unicast, and the address is in the address table, and if the associated interface is not the interface on which the frame arrived, forward the frame out the one correct port.
5.Otherwise, filter (do not forward) the frame.

this from http://ciscotests.org/ccna.php?part=3&page=2 odom, lammele, et al

according to odom:
paraphrase liberty alert

frame came in port E0, filter because destination is on port E0...
frame came in port E0, forward because destination is off  port E1

filtering happens when the frame gets dropped, and forwarding happens when the frame is shipped to a port besides the one on which the frame was received...

switch happens...