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Someone feel free to correct me if I'm wrong but i understood it as the arp request is flooded as it is a broadcast sent to the all Fs address, where as unknown unicast flooding is a unicast frame sent to a specific address that isn't in the switches mac table so it is flooded out all ports.
That's how I understand it as well.
Unknown unicast flooding is when a switch receives a frame with a specific destination MAC, but it doesn't have that destination in its address table. It floods the frame out all ports, if it sees a response it then know which port the device is connected to, it'll update its address table and future traffic won't be flooded.
So unicast flooding is just a specific type of flooding. Broadcast frames being sent out all ports would just be 'flooding'
There is no such thing as unknown unicast flooding. There is no such thing as known unicast flooding either. its just unicast flooding.
A switch only ever does four things: Learning Flooding Forwarding Filtering
If a switch receives a broadcast frame, which is to say a frame with a destination MAC of ffff.ffff.ffff, the switch will always perform the flood action.
If a switch receives a unicast frame, with a destination MAC address it knows the location of the switch will perform the forward action.
If a switch receives a unicast frame, with a destination MAC address it does NOT know the location of the switch will perform the flood action. This event is sometimes referred to as unicast flooding.
You can see an example of all this in this animation.
"Unknown Unicast Flooding" is definitely not the correct term. Because the Switch flooding the ARP is processing a broadcast frame (not Unicast).