For switches, I understand what forward data is and what filter data is but don't know what flood data is
A+ Network Devices Switches - What is Flood Data?
Thank you for the great question. Let's see if we can help with this one by identifying the switch functionality.
Switches essentially pass data between ports and when it is a from say devices A and B connected to ports A and B this is what we call forwarding. When device A sends a frame/data into the switche's port A the switch inspects the MAC address for the destination device which in this case is device B on port B. This address is referenced from a database the switch maintains for all of it's ports called a MAC table. This is forwarding, think of it as sending data between two ports. Now with filtering lets say that the communication from device A is a broadcast communication, this might be a good time to review the three basic types of communications:
Unicast- One to One
Broadcast- One to All
Multicast- One to a few/group
If the communication from device A is broadcast then the switch receives the frame/data, inspects the data, and sends it to ALL devices connected to the switch ports EXCEPT the one port that device A is connected to (filtering) as there is no need to send a broadcas (One to All) to the same device that sent the broadcast.
So now we have flooding, what the heck is that? Well imagine the last scenario in which device A sent a broadcast (One to All) communication. Because the intended target or destination is EVERY DEVICE, the switch sends the data out all ports (flooding) except the port the switch received the communication on (filtering). However this is only one scenario in which a switch will flood.
Another scenario is one in which the switch receives data, the switch inspects the data, looks to the MAC table and does find the destination MAC address. The switch will send the data out ALL ports. This is essentially flooding. This situation does not happen much with modern operating systems as the OSes will usually talk with the switch during or after the boot process making the switch aware of the MAC address. You will still see flooding on devices that do not talk that often with the switch like printers. These devices talk with the switch when they boot but then can sit idle for days and a switch will typically purge it's MAC table of old (stale) MAC addresses or put to it another way the switch clears it's database of device MAC addresses that do not talk with the switch after a certain period of time elapses. I hope this helps and please let me know!!