SDA Underlay and Overlay

Considering network of devices used for communication, there are edge networking devices connected to endpoint devices. They encapsulate or decapsulate data send by endpoints. There are also many networking devices lying between these edge devices which provide connectivity for edge devices.

The underlying network of devices that exist between edge networking devices is called Underlay.
They include switches, routers, cables and wireless links to create physical network.

Like GRE tunnel, path defined for packet to take (as specified by protocol used) can be though as a virtual connection between edge devices. This virtual path along with edge devices is collectively called Overlay.

Network implementing SDA also uses this concept of overlay and underlay. All switches in distribution and access layer used in SDA network are layer 3 switches. This network which uses layer 3 access layer switches is called Routed access layer. The virtual link that exists between edge networking devices in SDA network is called VXLAN.

In SDA network architecture, Overlay and Underlay are collectively known as SDA fabric and edge networking devices are called Fabric edge nodes.

Figure- SDA Underlay and Overlay

Location Identifier Separation Protocol :
In network implementing SDA, for identifying the location of endpoint devices and routes to reach them is learned through a different concept than traditional mac and ip address learning. These concepts are defined by Location Identifier Separation Protocol (LISP). Each endpoint devices are given a identification number called Endpoint Identifier (EID).

  • Fabric Edge nodes uses traditional mac or IP address method to identify endpoint devices. Then an EID is given to all endpoint devices.
  • A database called LISP map server contains EID list.
  • LISP map server also contains list to reach each EID via fabric edge node’s IP address. IP address of fabric edge node is called Routing location (RLOC). This list contains EID and matching RLOC.

Lisp map server

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