The following concepts apply to Layer 3 leaf-spine topologies:
- Each leaf switch connects to every spine switch in the topology.
- Servers, storage arrays, edge routers, and similar devices always connect to leaf switches in Layer 2 traffic between leaf and spine switches is routed. The Layer 3 Layer 2 boundary is at the leaf switches. Spine switches should be dedicated purely for switching packets in Layer 3.
- Layer 3 topologies use two leaf switches at the top of each rack configured as a Virtual Link Trunking (VLT) pair. VLT allows all connections to be active while providing fault-tolerance. As administrators add racks to the data center, two leaf switches configured for VLT are added to each new rack.
- The total number of leaf-spine connections equals the number of leaf switches multiplied by the number of spine switches. To increase fabric bandwidth additional connections between leaf and spine switches can be implemented if the spine layer has the capacity for the added connections.
