Typical Two‑Layer Spine‑Leaf Topology and Its Scalability

Figure 4 shows a typical two‑layer spine and leaf topology.

Figure 4. Typical spine and leaf topology.

In this two‑layer Clos architecture, each lower‑layer switch (leaf) is connected to every top‑layer switch (spine) in a full‑mesh topology. The leaf layer consists of access switches that connect to servers and other devices, while the spine layer forms the network backbone interconnecting all leaf switches. Every leaf switch connects to each spine switch, and paths are chosen randomly to distribute traffic evenly across the top switches; a failure of a single spine switch only slightly reduces overall data‑center performance.

If link oversubscription occurs—i.e., generated traffic exceeds the aggregate capacity of active links—the capacity can be expanded simply by adding an additional spine switch and extending the uplinks to each leaf, thereby increasing inter‑layer bandwidth and reducing oversubscription. When port capacity becomes a bottleneck, new leaf switches can be added to each spine and the network configuration updated. This scalability streamlines the IT department’s network expansion process, and if lower‑layer switches and their uplinks are not oversubscribed, a non‑blocking architecture can be achieved.

For spine‑leaf architectures, regardless of which leaf a server connects to, its traffic must traverse the same number of devices to reach another server (unless the destination server resides on the same leaf). This keeps latency predictable, as traffic only needs to hop to one spine switch and then to another leaf switch to reach its destination.

Original source: Cisco White Paper

Article: http://jiagoushi.pro/node/1032

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