(snip)
Well, traffic demand and traffic allowed are different. If many hosts are able to send somewhere close to 100Mb, the switch will be forced to discard packets. Without flow control there is no way for the switch to slow down the traffic coming in.
On half duplex links, some switches will do flow control by forcing collisions on incoming packets, so at least the sending host knows that the packets aren't getting through.
This problem will only occur when there is enough traffic to more than fill the 100Mb uplink, which should be relatively rare. Maybe a group of machines all trying to backup data to a server at the same time would do it.
Somewhat similar to traffic meters on freeway onramps that limit the rate traffic can enter the freeway to allow the freeway to run faster. People aren't good at doing flow control without an external signal and the threat of a ticket.
-- glen