  The advent of software defined networking enables flexible, reliable and
feature-rich control planes for data center networks. However, the tight
coupling of centralized control and complete visibility leads to a wide range
of issues among which scalability has risen to prominence. To address this, we
present LazyCtrl, a novel hybrid control plane design for data center networks
where network control is carried out by distributed control mechanisms inside
independent groups of switches while complemented with a global controller. Our
design is motivated by the observation that data center traffic is usually
highly skewed and thus edge switches can be grouped according to traffic
locality. LazyCtrl aims at bringing laziness to the global controller by
dynamically devolving most of the control tasks to independent switch groups to
process frequent intra-group events near datapaths while handling rare
inter-group or other specified events by the controller. We implement LazyCtrl
and build a prototype based on Open vSwich and Floodlight. Trace-driven
experiments on our prototype show that an effective switch grouping is easy to
maintain in multi-tenant clouds and the central controller can be significantly
shielded by staying lazy, with its workload reduced by up to 82%.
