Data center and cloud architectures continue to evolve to address the needs of large-scale multi-tenant data centers and clouds. These needs are centered around seven dimensions: scalability in computing, storage, and bandwidth, scalability in network services, efficiency in resource utilization, agility in service creation, cost efficiency, service reliability, and security. This article focuses on the first five dimensions as they pertain to networking. Large data centers are targeting support for tens of thousands of servers, exabytes of storage, terabits per second of traffic, and tens of thousands of tenants. In a data center, server and storage resources are interconnected with packet switches and routers that provide for the bandwidth and multi-tenant virtual networking needs. Data centers are interconnected across the wide area network via routing and transport technologies to provide a pool of resources, known as the cloud. High-speed optical interfaces and dense wavelength-division multiplexing optical transport are used to provide for high-capacity transport intra- and inter-datacenter. This article reviews various switching, routing, and optical transport technologies, and their applicability in addressing the networking needs of large-scale multi-tenant data centers.