Implementing SDN

[Sascha2020]

Hey guys, 
i'm currently trying to completely understand how the Cisco SDN-Solutions are implemented in an existing network, and i'm a bit confused by all the information online about this topics at Cisco. Maybe you guys can enlighten me.
So if i got that right, a cisco SDN-Solution, like SD-Access or ACI, implements an overlay-network on top of an existing underlay (Layer-3?) network. And in that overlay-network, many different services and protocols in all different logical layers can be implemented. 

Are my assumptions correct so far?

And how is it prevented that this creates a bottleneck if all the implemented protocols and services in the overlay-network are limited to the specifications of the underlay-network? Like e.g. everything from the overlay-network still get encapsulated with IP-Protocol on Layer-3 in the underlay network?

please excuse my bad english and my may stupid questions, the more i research online, the more confused i am.

Thanks in advance and best regards

Sascha

[balaji.bandi]

High level as per my learning, you need to Plan Underlay as strong as big you can to meet Overlay.

With out proper Undelay - there is no overlay - simple.

That is the reason going to SD-Access or SDN / ACI.

You need to plan correctly your underlay with right  Banwidth / Interface and Protoco, once this is done, its very hard to change later stage because overlay only stable based on underlay.

Deep dive information can be find here :

https://www.cisco.com/c/en/us/td/docs/solutions/CVD/Campus/SD-Access-Distributed-Campus-Deployment-Guide-2019JUL.html