Data centers were the first large-scale SDN deployments. Google, Facebook, and Microsoft built custom SDN solutions.
Challenges SDN solves:
VM mobility. VMs move between hosts. SDN moves policies with them.
Scale. Thousands of switches, millions of flows. Manual configuration is impossible.
Multi-tenancy. Cloud providers need isolation between customers.
Common patterns:
Spine-leaf topology with SDN managing ECMP
VXLAN overlay networks with controller managing tunnels
Microsegmentation with per-workload firewall policies
Major cloud providers run millions of VMs on SDN-controlled networks.