Those are the ones I am talking about. The problem is that you have to work out where the packet is going whilst the header comes in, else you delay the packet. With high end switches, delaying a lot of packets means a lot of storage for that bubble of data. So the problem is how fast you can look up a destination MAC and map it to an output port. I suppose if the downstream switch tells you to stall transmit then you have no choice but to buffer the packet, and gddr would be good for that.