The problem with traditional networks built using the spanning-tree protocol or layer-3 routed core networks is that a single "best path" is chosen from a set of alternative paths. All data traffic takes that "best path" until the point that it gets congested then packets are dropped. The alternative paths are not utilized because they topology algorithm deemed them to be less desirable or removed to prevent loops from forming. There is a desire to migrate away from using spanning-tree while still maintaining a loop-free topology yet utilizing all the multiple redundant links. If we could use a method of Equal-Cost Multi-Path(ECMP) routing, then performance could increase and the network would have better resiliency in the event of a link failure or a single switch failure.
Clos networks have now made their second reappearance in modern data center switching topologies. However, this time, rather than being a fabric within a single device, the Clos network now manifests itself in the way that the switches are interconnected. Now data center networks are comprised of top-of-rack switches and core switches. The top of rack (ToR) switches are the leaf switches and they are attached to the core switches which represent the spine. The leaf switches are not connected to each other and spine switches only connect to the leaf switches (or an upstream core device). In this Spine-Leaf architecture, the number of uplinks from the leaf switch equals the number of spine switches. Similarly, the number of downlinks from the spike equal the number of leaf switches. The total number of connections is the number of leaf switches multiplied by the number of spine switches (in this diagram 8 X 4 = 32 links).
In this Clos topology, every lower-tier switch is connected to each of the top-tier switches in a full-mesh topology. If there isn't any oversubscription taking place between the lower-tier switches and their uplinks, then a non-blocking architecture can be achieved. The advantage of the Clos network is you can use a set of identical and inexpensive devices to create the tree and gain high performance and resilience that would otherwise cost must more to construct. To prevent any one uplink path from being chosen, the path is randomly chosen so that the traffic load is evenly distributed between the top-tier switches. If one of the top tier switches were to fail, it only slightly degrades performance through the data center.
Examples of Data Center Clos Networks
There are examples of Clos networks is many of the data center fabric architectures from switch manufacturers. Transparent Interconnect of Lots of Links (TRILL) is a layer-2 data center protocol that creates flat networks on top of a layer-3 routed network for the purposes of simplified server networking. TRILL allow for multiple paths to be used in a redundant Clos Network architecture and removes the need for spanning tree protocol and its blocked alternative links. Many vendors have implemented their own versions of TRILL.
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