Reference clock addresses are of the form 127.127. The server command is used to configure a reference clock, where the address argument in that command is the clock address. The key , version and ttl options are not used for reference clock support. The prefer option can be useful to persuade the server to cherish a reference clock with somewhat more enthusiasm than other reference clocks or peers. Further information on this option can be found in the Mitigation Rules and the prefer Keyword page.
Some reference clocks require additional options to further specify the operation.
The stratum of a reference clock is by default zero. Since the xntpd daemon adds one to the stratum of each peer, a primary server ordinarily displays stratum one. In order to provide engineered backups, it is often useful to specify the reference clock stratum as greater than zero. The stratum option is used for this purpose. The refid option is used for this purpose.