A customer migrates from a traditional Layer 2 data center network into a new SDN-based spine- and-leaf VXLAN data center within the same location Its applications cannot be readdressed and the…

Layer 3 links only would impose a routing boundary between the legacy and new networks, requiring applications to be readdressed into new subnets - which the customer explicitly cannot do.

Adding Layer 3 routed links alongside the Layer 2 trunk to border leaf switches introduces unnecessary routing complexity that could force IP boundary decisions and is not required when Layer 2 extension alone satisfies the migration constraint.

Connecting external or legacy networks to spine switches violates the spine-and-leaf design principle - spines exist solely to interconnect leaf switches and are not the correct attachment point for outside networks or migration traffic.

Border leaf switches in a spine-and-leaf VXLAN architecture are specifically designed as the attachment point for external networks and legacy environments. Extending a Layer 2 trunk from the legacy network to the border leaf preserves the original VLANs and IP subnets, so applications keep their existing IP addresses throughout the migration. This design supports a phased approach where workloads move incrementally from the legacy network into the VXLAN fabric without any readdressing.