what is underlay and overlag in ACI
Fabric is broken down into :
Underlay - Physical network of devices and connections.
Overlay - Virtual Network built on top of physical network.
For Overlay, we use VXLANs
VXLAN - Virtual Extensible LAN is a virtualization technology. It performs Network Virtualization over Layer 3.
VXLAN is going to encapsulate a layer 2 frame into UDP packet and then transmits it over L3 devices.