VDA Registers to DDCs that have been removed from the site and becomes unregistered

1. Install CitrixGroupPolicyManagement(_x64).exe locating at XA/XD installation media to AD first, then

• Open Group Policy Management in AD
• Navigate to the OU you want to create a Policy
• Create a GPO with name ListofDDCs and link it as below screenshot.
• Right click ListofDDCs  and edit it
• in the popped Group Policy Management Editor, Click Citrix Polices under Computer Configuration->Policies to view Citrix Computer Policies.

2. Click Edit in Citrix Computer Policies and add FQDN of newly added DDC under Policy named Controllers

3.Run gpupdate /force in the VMs or restart the VMs in the OU to make the Policy take effect. Check the registry at HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Citrix\VirtualDesktopAgent to make sure that the policy is correctly applied.

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Problem Cause

Customer configures DDCs by modifying control panel or change HKLM\Software\Citrix\VirtualDeskopAgent\ListofDDcs directly for initial vda registration, after Upgrade the site from low version to higher version, customer add another two DDCs into the site and remove old ones,if customer didn't make any changes to the VDA, VDA can register to new DDC by using VDA cache file updated by auto update of controllers policy who has the highest priority after initial registration which is enabled by default on DDC policy even though the DDCs listed in control panel or registry key are old ones. 

But once customer make some changes to HKLM\Software\Citrix\VirtualDeskopAgent\ListofDDcs, and set wrong ddcs list, VDA will take this as an initial registration and will use the modified DDC to register which leads to registration failure. 

Policy based method for configuring Controller or Cloud Connector addresses on a VDA has highest priority for initial VDA registration and will be recorded in VDA cache file as NonAutoListofDDCs monitored by auto update of controllers policy, so when you move a VDA to another site, it is recommended to set correct DDCs using AD policy to make sure that highest auto update of controllers policy together with its exceptions can guarantee VDA is registered all the time. 
 

https://docs.citrix.com/en-us/citrix-virtual-apps-desktops/manage-deployment/vda-registration.html#auto-update