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In a Layer 3 leaf-spine network, traffic between leafs and spines is routed. The Layer 2/3 boundary is at the leaf switches. Spine switches are never connected to each other in a Layer 3 topology. Equal cost multipath routing (ECMP) is used to load balance traffic across the Layer 3 network. Connections within racks from hosts to leaf switches are Layer 2. Connections to external networks are made from a pair of edge or border leafs, as shown in the following figure:
Figure 12. Layer 3 leaf-spine network
In a Layer 2 leaf-spine network, traffic between leafs and spines is switched (except for a pair of edge leafs), as shown in the following figure. VLT is used for multipathing and load-balancing traffic across the Layer 2 leaf-spine fabric. Connections from hosts to leaf switches are also Layer 2.
For connections to external networks, Layer 3 links are added between the spines and a pair of edge leafs.
Figure 13. Layer 2 leaf-spine network