Cisco SDA - The Underlay

An underlay network is the underlying physical infrastructure of a network typically built from a number of routers and switches with a layer 3 routing protocol. When building an underlay foundation for a SDA network the recommendation is to implement a routed access model. This prevents the need for protocols such as STP and HSRP. The network is made simpler while optimising reconvergence and all available links via non-blocking ECMP (equal cost multipathing).

Routed access = No layer 2 links = no STP = No reconvergence and use of all available links.

DNA-C is able to automatically discover, provision, and deploy your network underlay from scratch using best practices from Cisco's Validated Design. It uses IS-IS or OSPF for routing. This is known as 'LAN Automation'.

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Sources:
https://www.cisco.com/c/dam/en/us/products/se/2018/1/Collateral/nb-06-software-defined-access-ebook-en.pdf

https://www.ciscolive.com/c/dam/r/ciscolive/emea/docs/2019/pdf/BRKCRS-2810.pdf