Clos topologies have been widely adopted for large-scale data center networks (DCNs), but it has been difficult to support incremental expansions for Clos DCNs. Some prior work has claimed that the structure of Clos topologies hinders incremental expansion.
We demonstrate that it is indeed possible to design expandable Clos DCNs, and to expand them while they are carrying live traffic, without incurring packet loss. We use a layer of patch panels between blocks of switches in a Clos DCN, which makes physical rewiring feasible, and we describe how to use integer linear programming (ILP) to minimize the number of patch-panel connections that must be changed, which makes expansions faster and cheaper. We also describe a block-aggregation technique that makes our ILP approach scalable. We tested our "minimal-rewiring" solver on two kinds of fine-grained expansions using 2250 synthetic DCN topologies, and found that the solver can handle 99% of these cases while changing under 25% of the connections. Compared to prior approaches, this solver (on average) reduces the average number of "stages" per expansion from 4 to 1.29, and reduces the number of wires changed by an order of magnitude or more—a significant improvement to our operational costs, and to our exposure (during expansions) to capacity-reducing faults.
