At FB/AMZN scale its really tough to accurately emulate/virtualize the network, these are deep clos fabrics with LOTS OF STATE. At best you can get a sense of w

At FB/AMZN scale its really tough to accurately emulate/virtualize the network, these are deep clos fabrics with LOTS OF STATE. At best you can get a sense of what protocols might/should do, but at the end of the day the state of the real environment isn't present. So that micro-loop you created with your new policy that you observed in the lab turns in to a wave of micro-loops in real life because some random router was slow to update due to load.