topofrack

Top of rack (ToR) refers to a network design approach in data centers where a switch is placed at the top of each server rack. A ToR switch connects directly to the servers within the same rack via short Ethernet cables and provides uplink connectivity to the aggregation or spine layer of the data center network. This approach contrasts with older designs such as end-of-row (EoR) or middle-of-row (MoR), where switching occurs farther from the servers.

In a ToR-based fabric, each rack houses a ToR switch, commonly in a 1U or 2U form factor. Servers connect to the ToR with copper or fiber cables, and the ToR’s uplinks connect to a spine or aggregation layer, often within a leaf-spine topology. Modern ToR switches support a range of Ethernet speeds, including 10, 25, 40, and 100 Gigabit Ethernet, enabling scalable connectivity for compute, storage, and virtualization requirements.

Historically, ToR has become standard in many hyperscale and enterprise data centers since the late 2000s, as networks evolved to accommodate rapid server growth and higher bandwidth demands. The approach aims to reduce cabling complexity within aisles, improve manageability, and enable modular expansion by adding more racks with their own switches rather than reconfiguring a central switch fabric.

Benefits of ToR include reduced intra-rack cabling, easier fault isolation, and scalable growth by adding racks with their own switches. Challenges include the need for careful capacity planning to avoid uplink oversubscription, higher equipment counts, and the ongoing management of distributed switching across many devices. Major vendors offering ToR solutions include Cisco, Arista, Juniper, Huawei, Dell EMC, and HPE.