Changes you make to a machine or subnet's server preferences normally do not take effect on a given machine until that machine updates it nis_cachemgr data. When the nis_cachemgr of a machine updates its server-use information depends on whether the machine is obtaining its server preferences from a global client_info table or a local /var/nis/client_info file (see NIS+ Global Table or Local File).
Global table. The cache managers of machines obtaining their server preferences from global tables update their server preferences whenever the machine is booted or whenever the Time-to-live (TTL) value expires for the client_info table. By default, this TTL value is 12 hours, but you can change that as described in Changing the Time-to-Live of an NIS+ Object.
Local file. The cache managers of machines obtaining their server preferences from local files update their server preferences every 12 hours or whenever you run nisprefadm to change a server preference. (Rebooting the machine does not update the cache manager's server preference information.)
However, you can force server preference changes to take effect immediately by running nisprefadm with the -F option. The -F option forces nis_cachemgr to immediately update its information. See Putting NIS+ Server Preferences Into Immediate Effect for details.