Leaf-spine is a network topology in which a series of switches form the access layer.
Leaf-spine is an alternate to the three-layer core/aggregation/access network architecture. The leaf switches mesh into the spine, which is a series of several high-throughput layer 3 switches with high port density. Spine switches are essentially the core of the architecture, whereas leaf switches are the access layer that delivers network connection points for servers. Leaf switches also provide uplinks to spine switches.
Every leaf switch connects to every switch in the network fabric; no matter which leaf switch a server is connected to, it has to cross the same number of devices every time it connects to another server. The only exception is when the other server is on the same leaf. Latency is therefore minimized to an acceptable level because each payload only has to travel to a spine switch and another leaf switch to reach its endpoint.
A leaf-spine topology can be layer 2 or layer 3 — the links between the leaf and spine layer can be switched or routed. In a layer 2 leaf-spine design, Transparent Interconnection of Lots of Links or shortest path bridging takes the place of spanning-tree. All hosts are linked to the fabric and offer a loop-free route to their Ethernet MAC address through a shortest-path-first computation. In a layer 3 design, each link is routed, and it is most efficient when virtual local area networks are sequestered to individual leaf switches or when a network overlay, like VXLAN, is working.
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