By default, Primary, Reference, and Single Reference time servers use the Service Advertising Protocol (SAP) to announce their presence on the network.
Primary and Reference time servers use the SAP information to determine which other servers to poll in order to determine the network time.
Secondary time servers use the SAP information to choose a time server to follow.
An advantage of the SAP method is that it allows for quick installation without regard to the network layout. It also allows automatic reconfiguration if operating modes are changed or if new servers are added to the network.
Note that the SAP method might generate additional network traffic. The SAP method might also be disruptive in large network environments where ``test'' servers come and go, especially if the test server is configured as a time source (Single Reference, Reference, or Primary time server).