Some situations require you remove server roles on particular server or even remove the whole Skype for Business infrastructure in case you decided to move all users and services to the cloud. Even if you already moved all users to the cloud simple turning servers off won’t be enough.
What is Skype for Business Hybrid? Skype for Business Hybrid is two independent federated domains with a shared SIP address space. The sharing of the namespace allows administrators to move users between the two deployments with their data and redirect sign-in requests to the correct deployment. In any Hybrid configuration, the on premises deployment is the source of authority for that service. Therefore, sign-in requests will come to the on premises Front End servers using DNS lyncdiscover and SRV records. The on premises Front End server redirects requests to the cloud service from the discovery phase in case the user’s SIP identity and service lives (courtesy of AD) in Office 365. This allows the user’s client to sign in directly to the online tenant using Microsoft Office 365 URLs.
Have a look at the following diagrams, illustrating Sign In workflows for Internal and External users:
