Clos networks were first created in the mid-1950s as a method to switch telephone calls. Clos networks evolved into crossbar topologies and eventually into chassis-based Ethernet switches using a crossbar switching fabric. Now Clos networks are being used in modern data center networking architectures to achieve high performance and resiliency. This concept has been around for many years and it is now a key architectural model for data center networking. It is fascinating how concepts reemerge again and again in the history of networking.
Origin of the Clos Network
Charles Clos was a researcher at Bell Laboratories in the 1950s. He published a paper titled "A Study of Non-blocking Switching Networks" in the Bell System Technical Journal in 1953. In this paper he described how telephone calls could be switched with equipment that used multiple stages of interconnection to allow the calls to be completed. The switching points in the topology are called crossbar switches. Clos networks were designed to be a three-stage architecture, an ingress stage, a middle stage, and an egress stage. The concept is that there are multiple paths for the call to be switched through the network so that calls will always be connected and not "blocked" by another call. The term fabric came about later because the pattern of links looks like threads in a woven piece of cloth.
Clos Network within Network Switches
Clos networks made a reappearance many years later in the 1990s when early Ethernet switches were being developed. In order to create connectivity where any Ethernet interface on a switch could send Ethernet frames to any other interface on that switch, there needed to be a similar crossbar matrix of connectivity within the switch. The number of interfaces in the switch governed how large the crossbar fabric needed to be. When modular chassis-based network switches were developed, the crossbar switching fabric needed to grow to accommodate faster interface speeds. The crossbar fabric was provided by the supervisor module combined with the wiring between cards within the chassis.
Crossbar fabrics fell out of favor because they were subject to Head Of Line (HOL) blocking due to input queue limitations. Over time, Ethernet switches were developed that had input and output queues on all the interfaces. Modern Ethernet switches have more advanced fabric technologies, output queuing and priority-based flow control so they can now achieve non-blocking performance. With these technical enhancements, switches can now support guaranteed bandwidth connectivity for protocols like Fiber Channel over Ethernet (FCoE) using 10 Gigabit Ethernet links.
Data Center Switching Using Clos Architecture
Over the years, networks started to use the "fat tree" model of connectivity using the core - distribution - access architecture. In order to prevent oversubscription, the link speeds got progressively higher as you reached the core. For example, the access links to servers or desktops might have historically been 100Mbps Fast Ethernet links, the uplinks to the distribution switches might have been 1Gbps Ethernet links, and the uplinks from there to the core would have been 4X1Gbps port channels.
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