说明：  This is because IP addresses are location-dependent and change with host mobility and VM migration. Network configurations and policies, such as access control policies, specified by IP addresses require frequent updates that impose a large administrative burden. As an example, Google’s globally distributed scales up to millions of machines across hundreds of data centers [10] that are interconnected in layer 3. Using a scalable layer-2 networking technology, management complexity and cost of such huge networks could be significantly reduced.