Maps have different rates of change. For instance, some might not change for months at a time, such as protocols.byname among the default maps and auto_master among the nondefault maps; but passwd.byname can change several times a day. Scheduling map transfer using the crontab command allows you to set specific propagation times for individual maps.
To periodically run ypxfr at a rate appropriate for the map, the root crontab file on each slave server should contain the appropriate ypxfr entries. ypxfr contacts the master server and transfers the map only if the copy on the master server is more recent than the local copy.