When auto-update is enabled and you install a Virtual Delivery Agent (VDA), the VDA attempts to register with one of the Delivery Controller values you specified when you installed the VDA. The installer writes the Controller information you specify during VDA installation to the ListOfDDCs registry value.
After the VDA registers, the Controller with which it registered sends a list of the current Controller Fully Qualified Domain Names (FQDNs) and Security IDs (SIDs) to the VDA. The VDA writes this list to the auto-update persistent storage. Each Controller also checks the Site Configuration Database every 90 minutes for Controller information – if a Controller has been added or removed since the last check, or if a policy change has occurred, the Controller sends updated lists to its registered VDAs. The VDA will accept connections from all the Controllers in the most recent list it received.
If a VDA receives a list that does not include the Controller it is registered with (in other words, that Controller was removed from the Site), the VDA re-registers, choosing among the Controllers in the list. After a VDA registers or re-registers, it receives an updated list.